qa: add file sync stuck test support

qa: add file sync stuck test support

This will test the file sync of a directory, 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 3db3b4e2a4b853192c5b30c9594947ba45f96e03)

Conflicts:
	qa/tasks/cephfs/test_misc.py