os/FileStore: getattr() should return 0 if success

os/FileStore: getattr() should return 0 if success

* fix the test of ObjectStore/StoreTest.SimpleCloneTest, the expected
  return value of getattr() should be 0. otherwise it otherwise it fails
  on objectstore backends of "memstore" and "keyvaluestore".
* change FileStore::getattr() to return 0 on success
* tighten related tests in ceph_test_objectstore, to test the return value
  of getattr() using ASSERT_EQ(r, 0) instead of ASSERT_GE(r, 0)

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