LinuxKPI: Add sysctl(8) knob to control verbosity of WARN_ON's.

The purpose of this change is to reduce the amount of dmesg(8) noise when
VT switching after a panic.

Submitted by: Greg V <greg@unrelenting.technology>
Differential Revision: https://reviews.freebsd.org/D30174
Sponsored by: NVIDIA Networking

(cherry picked from commit 2390a1441effaba0e3d0f2f447f448aaf20428f1)