umount oops when remove blocklayoutdriver first

now pnfs client uses block layout, maybe we can remove
blocklayoutdriver first. if we umount later,
it can cause oops in unset_pnfs_layoutdriver.
because nfss->pnfs_curr_ld->clear_layoutdriver is invalid.

reproduce it:
 modprobe  blocklayoutdriver
 mount -t nfs4 -o minorversion=1 pnfsip:/ /mnt/
 rmmod blocklayoutdriver
 umount /mnt

then you can see following

CPU 0
Pid: 17023, comm: umount.nfs4 Tainted: GF          O 3.7.0-rc6-pnfs #1 VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform
RIP: 0010:[<ffffffffa04cfe6d>]  [<ffffffffa04cfe6d>] unset_pnfs_layoutdriver+0x1d/0x70 [nfsv4]
RSP: 0018:ffff8800022d9e48  EFLAGS: 00010286
RAX: ffffffffa04a1b00 RBX: ffff88000b013800 RCX: 0000000000000001
RDX: ffffffff81ae8ee0 RSI: ffff880001ee94b8 RDI: ffff88000b013800
RBP: ffff8800022d9e58 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff880001ee9400
R13: ffff8800105978c0 R14: 00007fff25846c08 R15: 0000000001bba550
FS:  00007f45ae7f0700(0000) GS:ffff880012c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: ffffffffa04a1b38 CR3: 0000000002c0c000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process umount.nfs4 (pid: 17023, threadinfo ffff8800022d8000, task ffff880006e48aa0)
Stack:
ffff8800105978c0 ffff88000b013800 ffff8800022d9e78 ffffffffa04cd0ce
ffff8800022d9e78 ffff88000b013800 ffff8800022d9ea8 ffffffffa04755a7
ffff8800022d9ea8 ffff880002f96400 ffff88000b013800 ffff880002f96400
Call Trace:
[<ffffffffa04cd0ce>] nfs4_destroy_server+0x1e/0x30 [nfsv4]
[<ffffffffa04755a7>] nfs_free_server+0xb7/0x150 [nfs]
[<ffffffffa047d4d5>] nfs_kill_super+0x35/0x40 [nfs]
[<ffffffff81178d35>] deactivate_locked_super+0x45/0x70
[<ffffffff8117986a>] deactivate_super+0x4a/0x70
[<ffffffff81193ee2>] mntput_no_expire+0xd2/0x130
[<ffffffff81194d62>] sys_umount+0x72/0xe0
[<ffffffff8154af59>] system_call_fastpath+0x16/0x1b
Code: 06 e1 b8 ea ff ff ff eb 9e 0f 1f 44 00 00 55 48 89 e5 53 48 83 ec 08 66 66 66 66 90 48 8b 87 80 03 00 00 48 89 fb 48 85 c0 74 29 <48> 8b 40 38 48 85 c0 74 02 ff d0 48 8b 03 3e ff 48 04 0f 94 c2
RIP  [<ffffffffa04cfe6d>] unset_pnfs_layoutdriver+0x1d/0x70 [nfsv4]
RSP <ffff8800022d9e48>
CR2: ffffffffa04a1b38
---[ end trace 29f75aaedda058bf ]---

Signed-off-by: fanchaoting<fanchaoting@cn.fujitsu.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org