ocfs2: fix panic due to ocfs2_wq is null

mount.ocfs2 failed when reading ocfs2 filesystem superblock encounters
an error.  ocfs2_initialize_super() returns before allocating ocfs2_wq.
ocfs2_dismount_volume() triggers the following panic.

  Oct 15 16:09:27 cnwarekv-205120 kernel: On-disk corruption discovered.Please run fsck.ocfs2 once the filesystem is unmounted.
  Oct 15 16:09:27 cnwarekv-205120 kernel: (mount.ocfs2,22804,44): ocfs2_read_locked_inode:537 ERROR: status = -30
  Oct 15 16:09:27 cnwarekv-205120 kernel: (mount.ocfs2,22804,44): ocfs2_init_global_system_inodes:458 ERROR: status = -30
  Oct 15 16:09:27 cnwarekv-205120 kernel: (mount.ocfs2,22804,44): ocfs2_init_global_system_inodes:491 ERROR: status = -30
  Oct 15 16:09:27 cnwarekv-205120 kernel: (mount.ocfs2,22804,44): ocfs2_initialize_super:2313 ERROR: status = -30
  Oct 15 16:09:27 cnwarekv-205120 kernel: (mount.ocfs2,22804,44): ocfs2_fill_super:1033 ERROR: status = -30
  ------------[ cut here ]------------
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 11753 Comm: mount.ocfs2 Tainted: G  E
        4.14.148-200.ckv.x86_64 #1
  Hardware name: Sugon H320-G30/35N16-US, BIOS 0SSDX017 12/21/2018
  task: ffff967af0520000 task.stack: ffffa5f05484000
  RIP: 0010:mutex_lock+0x19/0x20
  Call Trace:
    flush_workqueue+0x81/0x460
    ocfs2_shutdown_local_alloc+0x47/0x440 [ocfs2]
    ocfs2_dismount_volume+0x84/0x400 [ocfs2]
    ocfs2_fill_super+0xa4/0x1270 [ocfs2]
    ? ocfs2_initialize_super.isa.211+0xf20/0xf20 [ocfs2]
    mount_bdev+0x17f/0x1c0
    mount_fs+0x3a/0x160

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yi Li <[email protected]>
Reviewed-by: Joseph Qi <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Changwei Ge <[email protected]>
Cc: Gang He <[email protected]>
Cc: Jun Piao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/ocfs2/journal.c

@@ -217,7 +217,8 @@ void ocfs2_recovery_exit(struct ocfs2_super *osb)
 	/* At this point, we know that no more recovery threads can be
 	 * launched, so wait for any recovery completion work to
 	 * complete. */
-	flush_workqueue(osb->ocfs2_wq);
+	if (osb->ocfs2_wq)
+		flush_workqueue(osb->ocfs2_wq);
 
 	/*
 	 * Now that recovery is shut down, and the osb is about to be

fs/ocfs2/localalloc.c

@@ -377,7 +377,8 @@ void ocfs2_shutdown_local_alloc(struct ocfs2_super *osb)
 	struct ocfs2_dinode *alloc = NULL;
 
 	cancel_delayed_work(&osb->la_enable_wq);
-	flush_workqueue(osb->ocfs2_wq);
+	if (osb->ocfs2_wq)
+		flush_workqueue(osb->ocfs2_wq);
 
 	if (osb->local_alloc_state == OCFS2_LA_UNUSED)
 		goto out;