reiserfs: Avoid touching renamed directory if parent does not change

The VFS will not be locking moved directory if its parent does not
change. Change reiserfs rename code to avoid touching renamed directory
if its parent does not change as without locking that can corrupt the
filesystem.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>