Btrfs: fix crash caused by block group removal

If we remove a block group (because it became empty), we might have left
a caching_ctl structure in fs_info->caching_block_groups that points to
the block group and is accessed at transaction commit time. This results
in accessing an invalid or incorrect block group. This issue became visible
after Josef's patch "Btrfs: remove empty block groups automatically".

So if the block group is removed make sure we don't leave a dangling
caching_ctl in caching_block_groups.

Sample crash trace:

[58380.439449] BUG: unable to handle kernel paging request at ffff8801446eaeb8
[58380.439707] IP: [<ffffffffa03f6d05>] block_group_cache_done.isra.21+0xc/0x1c [btrfs]
[58380.440879] PGD 1acb067 PUD 23f5ff067 PMD 23f5db067 PTE 80000001446ea060
[58380.441220] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[58380.441486] Modules linked in: btrfs crc32c_generic xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd fscache sunrpc loop psmouse processor i2c_piix4 parport_pc parport pcspkr serio_raw evdev i2ccore thermal_sys microcode button ext4 crc16 jbd2 mbcache sr_mod cdrom ata_generic sg sd_mod crc_t10dif crct10dif_generic crct10dif_common virtio_scsi floppy ata_piix e1000 libata virtio_pci scsi_mod virtio_ring virtio [last unloaded: btrfs]
[58380.443238] CPU: 3 PID: 25728 Comm: btrfs-transacti Tainted: G        W      3.17.0-rc5-btrfs-next-1+ #1
[58380.443238] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
[58380.443238] task: ffff88013ac82090 ti: ffff88013896c000 task.ti: ffff88013896c000
[58380.443238] RIP: 0010:[<ffffffffa03f6d05>]  [<ffffffffa03f6d05>] block_group_cache_done.isra.21+0xc/0x1c [btrfs]
[58380.443238] RSP: 0018:ffff88013896fdd8  EFLAGS: 00010283
[58380.443238] RAX: ffff880222cae850 RBX: ffff880119ba74c0 RCX: 0000000000000000
[58380.443238] RDX: 0000000000000000 RSI: ffff880185e16800 RDI: ffff8801446eaeb8
[58380.443238] RBP: ffff88013896fdd8 R08: ffff8801a9ca9fa8 R09: ffff88013896fc60
[58380.443238] R10: ffff88013896fd28 R11: 0000000000000000 R12: ffff880222cae000
[58380.443238] R13: ffff880222cae850 R14: ffff880222cae6b0 R15: ffff8801446eae00
[58380.443238] FS:  0000000000000000(0000) GS:ffff88023ed80000(0000) knlGS:0000000000000000
[58380.443238] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[58380.443238] CR2: ffff8801446eaeb8 CR3: 0000000001811000 CR4: 00000000000006e0
[58380.443238] Stack:
[58380.443238]  ffff88013896fe18 ffffffffa03fe2d5 ffff880222cae850 ffff880185e16800
[58380.443238]  ffff88000dc41c20 0000000000000000 ffff8801a9ca9f00 0000000000000000
[58380.443238]  ffff88013896fe80 ffffffffa040fbcf ffff88018b0dcdb0 ffff88013ac82090
[58380.443238] Call Trace:
[58380.443238]  [<ffffffffa03fe2d5>] btrfs_prepare_extent_commit+0x5a/0xd7 [btrfs]
[58380.443238]  [<ffffffffa040fbcf>] btrfs_commit_transaction+0x45c/0x882 [btrfs]
[58380.443238]  [<ffffffffa040c058>] transaction_kthread+0xf2/0x1a4 [btrfs]
[58380.443238]  [<ffffffffa040bf66>] ? btrfs_cleanup_transaction+0x3d8/0x3d8 [btrfs]
[58380.443238]  [<ffffffff8105966b>] kthread+0xb7/0xbf
[58380.443238]  [<ffffffff810595b4>] ? __kthread_parkme+0x67/0x67
[58380.443238]  [<ffffffff813ebeac>] ret_from_fork+0x7c/0xb0
[58380.443238]  [<ffffffff810595b4>] ? __kthread_parkme+0x67/0x67

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>