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>

fs/btrfs/extent_io.c

@@ -1213,7 +1213,7 @@ static int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 	bio_ctrl->end_io_func = end_bio_extent_readpage;
 	begin_page_read(fs_info, page);
 	while (cur <= end) {
-		unsigned long this_bio_flag = 0;
+		enum btrfs_compression_type compress_type = BTRFS_COMPRESS_NONE;
 		bool force_bio_submit = false;
 		u64 disk_bytenr;
 
@@ -1238,11 +1238,11 @@ static int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 		BUG_ON(end < cur);
 
 		if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags))
-			this_bio_flag = em->compress_type;
+			compress_type = em->compress_type;
 
 		iosize = min(extent_map_end(em) - cur, end - cur + 1);
 		iosize = ALIGN(iosize, blocksize);
-		if (this_bio_flag != BTRFS_COMPRESS_NONE)
+		if (compress_type != BTRFS_COMPRESS_NONE)
 			disk_bytenr = em->block_start;
 		else
 			disk_bytenr = em->block_start + extent_offset;
@@ -1314,13 +1314,13 @@ static int btrfs_do_readpage(struct page *page, struct extent_map **em_cached,
 			continue;
 		}
 
-		if (bio_ctrl->compress_type != this_bio_flag)
+		if (bio_ctrl->compress_type != compress_type)
 			submit_one_bio(bio_ctrl);
 
 		if (force_bio_submit)
 			submit_one_bio(bio_ctrl);
 		ret = submit_extent_page(bio_ctrl, disk_bytenr, page, iosize,
-					 pg_offset, this_bio_flag);
+					 pg_offset, compress_type);
 		if (ret) {
 			/*
 			 * We have to unlock the remaining range, or the page