qa: unmount clients before deleting fs

Client unmount during test cleanup will hang if the file system was deleted.

Fixes: http://tracker.ceph.com/issues/38518
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit a20a61f)

qa/tasks/cephfs/test_misc.py

@@ -52,6 +52,9 @@ def test_root_rctime(self):
         self.assertGreaterEqual(rctime, t-10)
 
     def test_fs_new(self):
+        self.mount_a.umount_wait()
+        self.mount_b.umount_wait()
+
         data_pool_name = self.fs.get_data_pool_name()
 
         self.fs.mds_stop()