ptp: replace getnstimeofday64() with ktime_get_real_ts64()

getnstimeofday64() is deprecated and getting replaced throughout
the kernel with ktime_get_*() based helpers for a more consistent
interface.

The two functions do the exact same thing, so this is just
a cosmetic change.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>