ruxreset: Add an inline function to reset all the stats in rusage_ext

Use it in proc0_post to reset per-process CPU usage.

Suggested by: olce
Reviewed by: olce, kib
Differential Revision: https://reviews.freebsd.org/D54049

(cherry picked from commit 3f8ed605dbd0ee211ce8fe59c0c072b916d6311c)