scsi_sysfs: protect against double execution of __scsi_remove_device()

On some host errors storvsc module tries to remove sdev by scheduling a job
which does the following:

   sdev = scsi_device_lookup(wrk->host, 0, 0, wrk->lun);
   if (sdev) {
       scsi_remove_device(sdev);
       scsi_device_put(sdev);
   }

While this code seems correct the following crash is observed:

 general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
 RIP: 0010:[<ffffffff81169979>]  [<ffffffff81169979>] bdi_destroy+0x39/0x220
 ...
 [<ffffffff814aecdc>] ? _raw_spin_unlock_irq+0x2c/0x40
 [<ffffffff8127b7db>] blk_cleanup_queue+0x17b/0x270
 [<ffffffffa00b54c4>] __scsi_remove_device+0x54/0xd0 [scsi_mod]
 [<ffffffffa00b556b>] scsi_remove_device+0x2b/0x40 [scsi_mod]
 [<ffffffffa00ec47d>] storvsc_remove_lun+0x3d/0x60 [hv_storvsc]
 [<ffffffff81080791>] process_one_work+0x1b1/0x530
 ...

The problem comes with the fact that many such jobs (for the same device)
are being scheduled simultaneously. While scsi_remove_device() uses
shost->scan_mutex and scsi_device_lookup() will fail for a device in
SDEV_DEL state there is no protection against someone who did
scsi_device_lookup() before we actually entered __scsi_remove_device(). So
the whole scenario looks like that: two callers do simultaneous (or
preemption happens) calls to scsi_device_lookup() ant these calls succeed
for both of them, after that they try doing scsi_remove_device().
shost->scan_mutex only serializes their calls to __scsi_remove_device()
and we end up doing the cleanup path twice.

Signed-off-by: Vitaly Kuznetsov <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>