vmmon: mark expected switch fall through

Since v5.3-rc2, kernel is built with -Wimplicit-fallthrough by default
so that compiler warns about switch fall thgrough unless it is marked as
expected. Add comment to mark one existing fall through as expected.

Note: this function is no longer present in 15.1.0 and it is probably
dead code anyway as hash2() is only called from HashFunc_HashPage()
which does not seem to be called anywhere. But to keep the code as close
to upstream as reasonably possible, just mark the fall through and keep
both functions in place.

vmmon-only/include/hashFunc.h

@@ -251,6 +251,7 @@ static INLINE ub8 hash2(register const ub8 *k, /* the key */
   {
     /* c is reserved for the length */
   case  2: b+=k[1];
+	   /* fall through */
   case  1: a+=k[0];
     /* case 0: nothing left to add */
   }