qa: krbd_blkroset.t: eliminate a race in the open_count test

qa: krbd_blkroset.t: eliminate a race in the open_count test

Even at QD=1, dd may take less than 10 seconds to work its way to the
end of a 10M image, producing "No space left on device" error instead
of the expected "Operation not permitted" error which is supposed to
arise from the device getting marked read-only while opened.

Fixes: https://tracker.ceph.com/issues/74513
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>