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

sys/kern/subr_asan.c

@@ -257,6 +257,9 @@ kasan_mark(const void *addr, size_t size, size_t redzsize, uint8_t code)
 	size_t i, n, redz;
 	int8_t *shad;
 
+	if (__predict_false(!kasan_enabled))
+		return;
+
 	if ((vm_offset_t)addr >= DMAP_MIN_ADDRESS &&
 	    (vm_offset_t)addr < DMAP_MAX_ADDRESS)
 		return;