ceph: use current_kernel_time() to get request time stamp

ceph uses ktime_get_real_ts() to get request time stamp. In most
other cases, current_kernel_time() is used to get time stamp for
filesystem operations (called by current_time()).

There is granularity difference between ktime_get_real_ts() and
current_kernel_time(). The later one can be up to one jiffy behind
the former one. This can causes inode's ctime to go back.
Signed-off-by: N"Yan, Zheng" <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -1687,7 +1687,6 @@ struct ceph_mds_request *
 ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
 {
 	struct ceph_mds_request *req = kzalloc(sizeof(*req), GFP_NOFS);
-	struct timespec ts;
 
 	if (!req)
 		return ERR_PTR(-ENOMEM);
@@ -1706,8 +1705,7 @@ ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
 	init_completion(&req->r_safe_completion);
 	INIT_LIST_HEAD(&req->r_unsafe_item);
 
-	ktime_get_real_ts(&ts);
-	req->r_stamp = timespec_trunc(ts, mdsc->fsc->sb->s_time_gran);
+	req->r_stamp = timespec_trunc(current_kernel_time(), mdsc->fsc->sb->s_time_gran);
 
 	req->r_op = op;
 	req->r_direct_mode = mode;