osd/ReplicatedPG: requeue cache full waiters if no longer writeback

osd/ReplicatedPG: requeue cache full waiters if no longer writeback

If the cache is full, we block some requests, and then we change the
cache_mode to something else (say, forward), the full waiters don't get
requeued until the cache becomes un-full.  In the meantime, however, later
requests will get processed and redirected, breaking the op ordering.

Fix this by requeueing any full waiters if we see that the cache_mode is
not writeback.

Fixes: #8931
Signed-off-by: Sage Weil <sage@redhat.com>