btrfs: simplify lookup_data_extent()

After returning if ret <= 0 we know that ret > 0. No need to check it.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain>