qa: krbd_blkroset.t: update for separate hw and user read-only flags

qa: krbd_blkroset.t: update for separate hw and user read-only flags

Since kernel 5.12, hardware read-only state and user read-only
policy (BLKROGET/SET ioctls) are tracked separately in the block
layer.  As the purpose of our ->set_read_only() method was exactly
that, it was removed.

As a side effect, BLKROSET no longer returns EROFS on an attempt
to make a read-only mapping read-write with "blockdev --setrw".
The policy gets updated, but the device remains read-only as before
because the hardware (== mapping) state is controlled by the driver.

Fixes: https://tracker.ceph.com/issues/49858
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit d72fca26edcff49d203ed6fb940e0cf331e943dd)