btrfs: fix error handling in free_log_tree

When we hit an I/O error in free_log_tree->walk_log_tree during file system
shutdown we can crash due to there not being a valid transaction handle.

Use btrfs_handle_fs_error when there's no transaction handle to use.

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000060
  IP: free_log_tree+0xd2/0x140 [btrfs]
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
  Modules linked in: <modules>
  CPU: 2 PID: 23544 Comm: umount Tainted: G        W        4.12.14-kvmsmall #9 SLE15 (unreleased)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
  task: ffff96bfd3478880 task.stack: ffffa7cf40d78000
  RIP: 0010:free_log_tree+0xd2/0x140 [btrfs]
  RSP: 0018:ffffa7cf40d7bd10 EFLAGS: 00010282
  RAX: 00000000fffffffb RBX: 00000000fffffffb RCX: 0000000000000002
  RDX: 0000000000000000 RSI: ffff96c02f07d4c8 RDI: 0000000000000282
  RBP: ffff96c013cf1000 R08: ffff96c02f07d4c8 R09: ffff96c02f07d4d0
  R10: 0000000000000000 R11: 0000000000000002 R12: 0000000000000000
  R13: ffff96c005e800c0 R14: ffffa7cf40d7bdb8 R15: 0000000000000000
  FS:  00007f17856bcfc0(0000) GS:ffff96c03f600000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000060 CR3: 0000000045ed6002 CR4: 00000000003606e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   ? wait_for_writer+0xb0/0xb0 [btrfs]
   btrfs_free_log+0x17/0x30 [btrfs]
   btrfs_drop_and_free_fs_root+0x9a/0xe0 [btrfs]
   btrfs_free_fs_roots+0xc0/0x130 [btrfs]
   ? wait_for_completion+0xf2/0x100
   close_ctree+0xea/0x2e0 [btrfs]
   ? kthread_stop+0x161/0x260
   generic_shutdown_super+0x6c/0x120
   kill_anon_super+0xe/0x20
   btrfs_kill_super+0x13/0x100 [btrfs]
   deactivate_locked_super+0x3f/0x70
   cleanup_mnt+0x3b/0x70
   task_work_run+0x78/0x90
   exit_to_usermode_loop+0x77/0xa6
   do_syscall_64+0x1c5/0x1e0
   entry_SYSCALL_64_after_hwframe+0x42/0xb7
  RIP: 0033:0x7f1784f90827
  RSP: 002b:00007ffdeeb03118 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
  RAX: 0000000000000000 RBX: 0000556a60c62970 RCX: 00007f1784f90827
  RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556a60c62b50
  RBP: 0000000000000000 R08: 0000000000000005 R09: 00000000ffffffff
  R10: 0000556a60c63900 R11: 0000000000000246 R12: 0000556a60c62b50
  R13: 00007f17854a81c4 R14: 0000000000000000 R15: 0000000000000000
  RIP: free_log_tree+0xd2/0x140 [btrfs] RSP: ffffa7cf40d7bd10
  CR2: 0000000000000060

Fixes: 681ae50917 ("Btrfs: cleanup reserved space when freeing tree log on error")
CC: <stable@vger.kernel.org> # v3.13
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -3193,9 +3193,12 @@ static void free_log_tree(struct btrfs_trans_handle *trans,
 	};
 
 	ret = walk_log_tree(trans, log, &wc);
-	/* I don't think this can happen but just in case */
-	if (ret)
-		btrfs_abort_transaction(trans, ret);
+	if (ret) {
+		if (trans)
+			btrfs_abort_transaction(trans, ret);
+		else
+			btrfs_handle_fs_error(log->fs_info, ret, NULL);
+	}
 
 	while (1) {
 		ret = find_first_extent_bit(&log->dirty_log_pages,