redzone: Raise a compile error if KASAN is configured

redzone(9) does some munging of the allocation to insert redzones before
and after a valid memory buffer, but KASAN does not know about this and
will raise false positives if both are configured.  Until this is fixed,
do not allow both to be configured.  Note that KASAN provides similar
checking on its own but currently does not force the creation of
redzones for all UMA allocations; this should be addressed as well.

Sponsored by:	The FreeBSD Foundation

sys/vm/redzone.c

@@ -37,6 +37,10 @@ __FBSDID("$FreeBSD$");
 
 #include <vm/redzone.h>
 
+#ifdef KASAN
+#error KASAN and DEBUG_REDZONE cannot be configured together
+#endif
+
 static SYSCTL_NODE(_vm, OID_AUTO, redzone, CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
     "RedZone data");
 static u_long redzone_extra_mem = 0;