btrfs: rename err to ret in btrfs_rmdir()

[ Upstream commit c3a1cc8f ]

Unify naming of return value to the preferred way.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: c466e33e ("btrfs: propagate last_unlink_trans earlier when doing a rmdir")
Signed-off-by: Sasha Levin <sashal@kernel.org>