qa/tasks/cephfs: compare number with number

qa/tasks/cephfs: compare number with number

to address following failure:
```
2020-04-05T12:25:30.997 INFO:tasks.cephfs_test_runner:Traceback (most recent call last):
2020-04-05T12:25:30.997 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_misc.py", line 50, in test_root_rctime
2020-04-05T12:25:30.998 INFO:tasks.cephfs_test_runner:    self.assertGreaterEqual(rctime, t-10)
2020-04-05T12:25:30.998 INFO:tasks.cephfs_test_runner:  File "/usr/lib/python3.5/unittest/case.py", line 1215, in assertGreaterEqual
2020-04-05T12:25:30.998 INFO:tasks.cephfs_test_runner:    if not a >= b:
2020-04-05T12:25:30.999 INFO:tasks.cephfs_test_runner:TypeError: unorderable types: str() >= float()
```

Signed-off-by: Kefu Chai <kchai@redhat.com>