KASAN: Disable checking before triggering a panic

KASAN hooks will not generate reports if panicstr != NULL, but then
there is a window after the initial panic() call where another report
may be raised. This can happen if a false positive occurs; to simplify
debugging of such problems, avoid recursing.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit ea3fbe0707f9a02a29875966668b6f15284f335a)