loop: Do not call loop_unplug for not configured loop device.

In loop_unplug() function is expected that mapping is set
and lo->lo_backing_file is not NULL.

Unfortunately loop_set_fd() set the request queue unplug function,
but loop_clr_fd() doesn't clear that.

Loop device allows open of non-configured loop in some situations.
If the unplug on request queue is called, loop module oopses because
of missing lo_backing_file.

Simple reproducer:
	losetup /dev/loop0 /xxx
	losetup -d /dev/loop0
	dmsetup create x --table "0 1 linear /dev/loop0 0"

 EIP is at loop_unplug+0x1d/0x3b
 ...
  Call Trace:
   blk_unplug+0x57/0x5e
   dm_table_unplug_all+0x34/0x77 [dm_mod]
   destroy_inode+0x27/0x38
   generic_delete_inode+0xd5/0xd9
   iput+0x4b/0x4e
   dm_resume+0xca/0xfe [dm_mod]
   dev_suspend+0x143/0x165 [dm_mod]
   dm_ctl_ioctl+0x18e/0x1cf [dm_mod]
   dev_suspend+0x0/0x165 [dm_mod]
   dm_ctl_ioctl+0x0/0x1cf [dm_mod]
   vfs_ioctl+0x22/0x69
   do_vfs_ioctl+0x39d/0x3c7
   trace_hardirqs_on+0xb/0xd
   remove_vma+0x50/0x56
   do_munmap+0x21c/0x237
   sys_ioctl+0x2c/0x45
   sysenter_do_call+0x12/0x31

Several reports here
http://www.kerneloops.org/search.php?search=loop_unplug

Fix it by simply clear unplug function together with
removing of backing file.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>