tcp: LRO timestamps have lost their previous precision

Recently we had a rewrite to tcp_lro.c that was tested but one subtle change
was the move to a less precise timestamp. This causes all kinds of chaos
in tcp's that do pacing and needs to be fixed to use the more precise
time that was there before.

Reviewed by: mtuexen, gallatin, hselasky
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30695

(cherry picked from commit b45daaea95abd8bda52caaacf120f9197caab3e7)