btrfs: rename the this_bio_flag variable in btrfs_do_readpage

Rename this_bio_flag to compress_type to match the surrounding code
and better document the intent.  Also use the proper enum type instead
of unsigned long.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>