qa: do not test dir size equals rbytes

qa: do not test dir size equals rbytes

Avoiding this failure:

	2020-11-03T03:11:59.980 INFO:tasks.cephfs_test_runner:test_subvolume_resize_fail_quota_lt_used_size_no_shrink (tasks.cephfs.test_volumes.TestSubvolumes) ... FAIL
	2020-11-03T03:11:59.981 INFO:tasks.cephfs_test_runner:
	2020-11-03T03:11:59.981 INFO:tasks.cephfs_test_runner:======================================================================
	2020-11-03T03:11:59.981 INFO:tasks.cephfs_test_runner:FAIL: test_subvolume_resize_fail_quota_lt_used_size_no_shrink (tasks.cephfs.test_volumes.TestSubvolumes)
	2020-11-03T03:11:59.982 INFO:tasks.cephfs_test_runner:----------------------------------------------------------------------
	2020-11-03T03:11:59.982 INFO:tasks.cephfs_test_runner:Traceback (most recent call last):
	2020-11-03T03:11:59.982 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/git.ceph.com_ceph-c_wip-pdonnell-testing-20201102.231317/qa/tasks/cephfs/test_volumes.py", line 1310, in test_subvolume_resize_fail_quota_lt_used_size_no_shrink
	2020-11-03T03:11:59.982 INFO:tasks.cephfs_test_runner:    self.assertEqual(usedsize, susedsize)
	2020-11-03T03:11:59.982 INFO:tasks.cephfs_test_runner:AssertionError: 8912896 != 1

From: /ceph/teuthology-archive/pdonnell-2020-11-03_02:22:55-fs:volumes-wip-pdonnell-testing-20201102.231317-distro-basic-smithi/5585383/teuthology.log

Fixes: https://tracker.ceph.com/issues/23718
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>