btrfs: move the bi_sector assignment out of btrfs_add_compressed_bio_pages

Adding pages to a bio has nothing to do with the sector.  Move the
assignment to the two callers in preparation for cleaning up
btrfs_add_compressed_bio_pages.

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