qa: add filesystem sync stuck test support

qa: add filesystem sync stuck test support

This will test the sync of the filesystem, which maybe stuck for
at most 5 seconds. This was because the related code will wait
for all the unsafe requests to get safe reply from MDSes, but the
MDSes just think that it's unnecessary to flush the mdlog immediately
after early reply, and the mdlog will be flushed every 5 seconds
in the tick thread.

This should have been fixed in kclient and libcephfs by triggering
mdlog flush before waiting requests' safe reply.

Fixes: https://tracker.ceph.com/issues/55283
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit b6fc5480f6ba6352fa72062e1376d0dd6b9074cd)