Btrfs: join the transaction in __btrfs_setxattr

With selinux on we end up calling __btrfs_setxattr when we create an inode,
which calls btrfs_start_transaction().  The problem is we've already called
that in btrfs_new_inode, and in btrfs_start_transaction we end up doing a
wait_current_trans().  If btrfs-transaction has started committing it will wait
for all handles to finish, while the other process is waiting for the
transaction to commit.  This is fixed by using btrfs_join_transaction, which
won't wait for the transaction to commit.  Thanks,
Signed-off-by: Josef Bacik <jbacik@redhat.com>

fs/btrfs/xattr.c

@@ -98,7 +98,7 @@ int __btrfs_setxattr(struct inode *inode, const char *name,
 	if (!path)
 		return -ENOMEM;
 
-	trans = btrfs_start_transaction(root, 1);
+	trans = btrfs_join_transaction(root, 1);
 	btrfs_set_trans_block_group(trans, inode);
 
 	/* first lets see if we already have this xattr */