btrfs: ctree: Reduce one indent level for btrfs_search_old_slot()

Similar to btrfs_search_slot() done in previous patch, make a shortcut
for the level 0 case and allow to reduce indentation for the remaining
case.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>