kasan: Add corresponding sysctl knob for loader tunable

The loader tunable 'debug.kasan.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 db5d0bc868be669ed6588ebeccf8c02e76aabc41)
(cherry picked from commit 6f8ef4d6e44ee27a08c14ab6a892ffccf332bcf7)