proc0_post: Clear relevant thread stats directly

rufetch() has several other effects besides clearing these per-thread
stats most of which are explicitly discarded by the subsequent calls
to ruxreset(). Just clear the relevant stats directly instead.

Reviewed by: olce, kib, markj
Differential Revision: https://reviews.freebsd.org/D54050

(cherry picked from commit acd02443a952b1f5412d35d03f081ad48bae34d7)