qa: krbd_stable_pages_required.sh: move to stable_writes attribute

qa: krbd_stable_pages_required.sh: move to stable_writes attribute

bdi/stable_pages_required attribute was deprecated in 5.10 and now
always returns 0.  The replacement is queue/stable_writes.  (It is
also writeable, so we can simplify these test cases somewhat in the
future.)

Fixes: https://tracker.ceph.com/issues/48232
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 5adfc15b873bc16d698e7398d3ef2c2a46e8a9df)

Conflicts:
	qa/workunits/rbd/krbd_stable_writes.sh
- this file is much bigger in master than in nautilus