net: skbuff.h: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3505,13 +3505,19 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
 #define __it(x, op) (x -= sizeof(u##op))
 #define __it_diff(a, b, op) (*(u##op *)__it(a, op)) ^ (*(u##op *)__it(b, op))
 	case 32: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case 24: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case 16: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case  8: diffs |= __it_diff(a, b, 64);
 		break;
 	case 28: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case 20: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case 12: diffs |= __it_diff(a, b, 64);
+		 /* fall through */
 	case  4: diffs |= __it_diff(a, b, 32);
 		break;
 	}