btrfs: pass bio opf to rbio_add_io_page

Prepare for further refactoring by moving this initialization to a
single place instead of setting it in the callers.

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