kasan: disable kasan_mark() after a violation

Specifically, when we receive a violation and we're configured to panic,
kasan_enabled gets unset before we descend into panic(). At this point,
there's no longer any reason to allow marking as kasan_shadow_check() is
disabled -- we have some inherent risk of faulting or panicking if the
system's in a bad enough state with no benefit.

Reviewed by: markj
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36742