mlx5 M_TSTMP accuracy looses quite a bit of precision so lets fix it.

The way that the clock is synchronized between the system and the current mlx5 for the purposes of the M_TSTMP
being carried we loose a lot of precision. Instead lets change the math that calculates this to separate out
the seconds/nanoseconds and operate on the two values so we don't get overflow instead of just
shifting the value down and loosing precision.

Reviewed by: kib, hselasky
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D36327