libsas: Fix Kernel Crash in smp_execute_task

This crash was reported:

[  366.947370] sd 3:0:1:0: [sdb] Spinning up disk....
[  368.804046] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  368.804072] IP: [<ffffffff81358457>] __mutex_lock_common.isra.7+0x9c/0x15b
[  368.804098] PGD 0
[  368.804114] Oops: 0002 [#1] SMP
[  368.804143] CPU 1
[  368.804151] Modules linked in: sg netconsole s3g(PO) uinput joydev hid_multitouch usbhid hid snd_hda_codec_via cpufreq_userspace cpufreq_powersave cpufreq_stats uhci_hcd cpufreq_conservative snd_hda_intel snd_hda_codec snd_hwdep snd_pcm sdhci_pci snd_page_alloc sdhci snd_timer snd psmouse evdev serio_raw pcspkr soundcore xhci_hcd shpchp s3g_drm(O) mvsas mmc_core ahci libahci drm i2c_core acpi_cpufreq mperf video processor button thermal_sys dm_dmirror exfat_fs exfat_core dm_zcache dm_mod padlock_aes aes_generic padlock_sha iscsi_target_mod target_core_mod configfs sswipe libsas libata scsi_transport_sas picdev via_cputemp hwmon_vid fuse parport_pc ppdev lp parport autofs4 ext4 crc16 mbcache jbd2 sd_mod crc_t10dif usb_storage scsi_mod ehci_hcd usbcore usb_common
[  368.804749]
[  368.804764] Pid: 392, comm: kworker/u:3 Tainted: P        W  O 3.4.87-logicube-ng.22 #1 To be filled by O.E.M. To be filled by O.E.M./EPIA-M920
[  368.804802] RIP: 0010:[<ffffffff81358457>]  [<ffffffff81358457>] __mutex_lock_common.isra.7+0x9c/0x15b
[  368.804827] RSP: 0018:ffff880117001cc0  EFLAGS: 00010246
[  368.804842] RAX: 0000000000000000 RBX: ffff8801185030d0 RCX: ffff88008edcb420
[  368.804857] RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffff8801185030d4
[  368.804873] RBP: ffff8801181531c0 R08: 0000000000000020 R09: 00000000fffffffe
[  368.804885] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801185030d4
[  368.804899] R13: 0000000000000002 R14: ffff880117001fd8 R15: ffff8801185030d8
[  368.804916] FS:  0000000000000000(0000) GS:ffff88011fc80000(0000) knlGS:0000000000000000
[  368.804931] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  368.804946] CR2: 0000000000000000 CR3: 000000000160b000 CR4: 00000000000006e0
[  368.804962] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  368.804978] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  368.804995] Process kworker/u:3 (pid: 392, threadinfo ffff880117000000, task ffff8801181531c0)
[  368.805009] Stack:
[  368.805017]  ffff8801185030d8 0000000000000000 ffffffff8161ddf0 ffffffff81056f7c
[  368.805062]  000000000000b503 ffff8801185030d0 ffff880118503000 0000000000000000
[  368.805100]  ffff8801185030d0 ffff8801188b8000 ffff88008edcb420 ffffffff813583ac
[  368.805135] Call Trace:
[  368.805153]  [<ffffffff81056f7c>] ? up+0xb/0x33
[  368.805168]  [<ffffffff813583ac>] ? mutex_lock+0x16/0x25
[  368.805194]  [<ffffffffa018c414>] ? smp_execute_task+0x4e/0x222 [libsas]
[  368.805217]  [<ffffffffa018ce1c>] ? sas_find_bcast_dev+0x3c/0x15d [libsas]
[  368.805240]  [<ffffffffa018ce4f>] ? sas_find_bcast_dev+0x6f/0x15d [libsas]
[  368.805264]  [<ffffffffa018e989>] ? sas_ex_revalidate_domain+0x37/0x2ec [libsas]
[  368.805280]  [<ffffffff81355a2a>] ? printk+0x43/0x48
[  368.805296]  [<ffffffff81359a65>] ? _raw_spin_unlock_irqrestore+0xc/0xd
[  368.805318]  [<ffffffffa018b767>] ? sas_revalidate_domain+0x85/0xb6 [libsas]
[  368.805336]  [<ffffffff8104e5d9>] ? process_one_work+0x151/0x27c
[  368.805351]  [<ffffffff8104f6cd>] ? worker_thread+0xbb/0x152
[  368.805366]  [<ffffffff8104f612>] ? manage_workers.isra.29+0x163/0x163
[  368.805382]  [<ffffffff81052c4e>] ? kthread+0x79/0x81
[  368.805399]  [<ffffffff8135fea4>] ? kernel_thread_helper+0x4/0x10
[  368.805416]  [<ffffffff81052bd5>] ? kthread_flush_work_fn+0x9/0x9
[  368.805431]  [<ffffffff8135fea0>] ? gs_change+0x13/0x13
[  368.805442] Code: 83 7d 30 63 7e 04 f3 90 eb ab 4c 8d 63 04 4c 8d 7b 08 4c 89 e7 e8 fa 15 00 00 48 8b 43 10 4c 89 3c 24 48 89 63 10 48 89 44 24 08 <48> 89 20 83 c8 ff 48 89 6c 24 10 87 03 ff c8 74 35 4d 89 ee 41
[  368.805851] RIP  [<ffffffff81358457>] __mutex_lock_common.isra.7+0x9c/0x15b
[  368.805877]  RSP <ffff880117001cc0>
[  368.805886] CR2: 0000000000000000
[  368.805899] ---[ end trace b720682065d8f4cc ]---

It's directly caused by 89d3cf6 [SCSI] libsas: add mutex for SMP task
execution, but shows a deeper cause: expander functions expect to be able to
cast to and treat domain devices as expanders.  The correct fix is to only do
expander discover when we know we've got an expander device to avoid wrongly
casting a non-expander device.

Reported-by: Praveen Murali <pmurali@logicube.com>
Tested-by: Praveen Murali <pmurali@logicube.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>