Probabilly defaulting to KTR_GEN is not the right decision when KTR_MASK

is not defined at all because KTR_GEN is still a valid class and some
traces may fit in. Default to 0, instead, and block any tracing.

As long as this is a POLA violation (some thirdy-part code, even if
that may be a questionable choice, could be rely on that feature) a
MFC possibility might be carefully evaluated.

Sponsored by:	Sandvine Incorporated

sys/kern/kern_ktr.c

@@ -65,7 +65,7 @@ __FBSDID("$FreeBSD$");
 #endif
 
 #ifndef KTR_MASK
-#define	KTR_MASK	(KTR_GEN)
+#define	KTR_MASK	(0)
 #endif
 
 #ifndef KTR_CPUMASK