ext4: rename: split out helper functions

Cross rename (exchange source and dest) will need to call some of these
helpers for both source and dest, while overwriting rename currently only
calls them for one or the other.  This also makes the code easier to
follow.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>