qa/tasks/cephfs/test_scrub.py: use umount_wait to avoid ceph-fuse stuck

qa/tasks/cephfs/test_scrub.py: use umount_wait to avoid ceph-fuse stuck

If the ceph-fuse client need to flush the caps and does sync wait,
the umount() will just return successfully, then the netns container
will be destroyed and the network will not be reachable, but the
ceph-fuse daemon is still stucked and waiting for the flush caps ack.

This will cause the ceph-fuse daemon get stuck forever and if the
mds daemons get restarted, it will try to reconnect the clients,
but the stucked ceph-fuse daemnon won't reply to it, because it is
not reachable any more.

Fixes: https://tracker.ceph.com/issues/45665
Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit a7423b2286154a4403e586b5148984649f988a03)