ceph: stop using current_kernel_time()

ceph_mdsc_create_request() is one of the last callers of the
deprecated current_kernel_time() as well as timespec_trunc().

This changes it to use the timespec64 based interfaces instead,
though we still need to convert the result until we are ready to
change over req->r_stamp.

The output of the two functions, ktime_get_coarse_real_ts64() and
current_kernel_time() is the same coarse-granular timestamp,
the only difference here is that ktime_get_coarse_real_ts64()
doesn't overflow in 2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>