[SCSI] fix scsi_setup_command_freelist failure path race

Looks like that host_cmd_pool_mutex are necessary here.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi.c

@@ -323,13 +323,12 @@ int scsi_setup_command_freelist(struct Scsi_Host *shost)
 	return 0;
 
  fail2:
+	mutex_lock(&host_cmd_pool_mutex);
 	if (!--pool->users)
 		kmem_cache_destroy(pool->slab);
-	return -ENOMEM;
  fail:
 	mutex_unlock(&host_cmd_pool_mutex);
 	return -ENOMEM;
-
 }
 
 /**