block, drivers, fs: shrink bi_rw from long to int

We don't need bi_rw to be so large on 64 bit archs, so
reduce it to unsigned int.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>