btrfs: remove useless return value statement in split_node

At the point when we set 'ret = 0' it's guaranteed that the function is
going to return 0 so directly return 0. No functional changes.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>