btrfs: open code extent_set_compress_type helpers

The helpers extent_set_compress_type and extent_compress_type have
become trivial after previous cleanups and can be removed.

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