block: shrink bio size again

The recent ops split grew the bio by adding the new ioprio field.
Shrink it again by using a 16-bit field for the bi_flags value and
filling the holes near the beginning of the structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>