mtx: Make idle thread assertions more robust

Just print the pointer to the mutex instead of the name in case the
mutex is corrupted.

Reviewed by: olce, kib
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D49314

(cherry picked from commit 0ed104978ce55ecacf42bde4458acda0cc773940)