btrfs: replace offset_in_entry with in_range

No point in duplicating the functionality just use the generic helper
that has the same semantics.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>