kmsan: Add corresponding sysctl knob for loader tunable

The loader tunable 'debug.kmsan.disabled' does not have corresponding
sysctl MIB entry. Add it so that it can be retrieved, and sysctl -T
will also report it correctly.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42138

(cherry picked from commit 1d2b743784f7527a6840fe35ddb7e34cd41bc17a)