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>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30174
Sponsored by: NVIDIA Networking