nfp: avoid using getnstimeofday64()

getnstimeofday64 is deprecated in favor of the ktime_get() family of
functions. The direct replacement would be ktime_get_real_ts64(),
but I'm picking the basic ktime_get() instead:

- using a ktime_t simplifies the code compared to timespec64
- using monotonic time instead of real time avoids issues caused
  by a concurrent settimeofday() or during a leap second adjustment.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>