openvswitch: use ktime_get_ts64() instead of ktime_get_ts()

timespec is deprecated because of the y2038 overflow, so let's convert
this one to ktime_get_ts64(). The code is already safe even on 32-bit
architectures, since it uses monotonic times. On 64-bit architectures,
nothing changes, while on 32-bit architectures this avoids one
type conversion.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>