The rw_page users were not converted to use bio/req ops. As a result
bdev_write_page is not passing down REQ_OP_WRITE and the IOs will
be sent down as reads.

Can we just get rid of REQ_OP_WRITE enum for the ->rw_page interface
and pass a 'bool write'?  If not I'd prefer to avoid using op_is_write
as much as possible - it's a confusing interface if we really want
to do a switch on read vs write vs invalid.