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: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>