btrfs: move the compress_type check out of btrfs_bio_add_page

The compress_type can only change on a per-extent basis.  So instead of
checking it for every page in btrfs_bio_add_page, do the check once in
btrfs_do_readpage, which is the only caller of btrfs_bio_add_page and
submit_extent_page that deals with compressed extents.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>