ext4: rename: create ext4_renament structure for local vars

Need to split up ext4_rename() into helpers but there are too many local
variables involved, so create a new structure.  This also, apparently,
makes the generated code size slightly smaller.

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