Btrfs: clear current->journal_info on async transaction commit

Normally current->jouranl_info is cleared by commit_transaction.  For an
async snap or subvol creation, though, it runs in a work queue.  Clear
it in btrfs_commit_transaction_async() to avoid leaking a non-NULL
journal_info when we return to userspace.  When the actual commit runs in
the other thread it won't care that it's current->journal_info is already
NULL.
Signed-off-by: Sage Weil <sage@newdream.net>
Tested-by: Jim Schutt <jaschut@sandia.gov>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/transaction.c

@@ -1118,8 +1118,11 @@ int btrfs_commit_transaction_async(struct btrfs_trans_handle *trans,
 		wait_current_trans_commit_start_and_unblock(root, cur_trans);
 	else
 		wait_current_trans_commit_start(root, cur_trans);
-	put_transaction(cur_trans);
 
+	if (current->journal_info == trans)
+		current->journal_info = NULL;
+
+	put_transaction(cur_trans);
 	return 0;
 }