IPv6: Use ND_ICHECKMSG_U() to print an invalid version

Update a test output accordingly.

Author: fxlb

print-ip6.c

@@ -247,10 +247,7 @@ ip6_print(netdissect_options *ndo, const u_char *bp, u_int length)
 	}
 
 	ND_ICHECK_ZU(length, <, sizeof (struct ip6_hdr));
-	if (IP6_VERSION(ip6) != 6) {
-		ND_PRINT("version error: %u != 6", IP6_VERSION(ip6));
-		return;
-	}
+	ND_ICHECKMSG_U("version", IP6_VERSION(ip6), !=, 6);
 
 	ND_TCHECK_SIZE(ip6);
 	payload_len = GET_BE_U_2(ip6->ip6_plen);

tests/ipv6-bad-version.out

@@ -1,4 +1,4 @@
     1  15:15:01.278565 IP6 :: > ff02::1:ff76:6c14: ICMP6, neighbor solicitation, who has fe80::20c:29ff:fe76:6c14, length 24
-    2  15:15:01.279230 IP6 version error: 0 != 6
+    2  15:15:01.279230 IP6  [version 0 != 6] (invalid)
     3  15:15:02.391170 IP6 :: > ff02::1:ff76:6c14: ICMP6, neighbor solicitation, who has 1111:2222:3333:4444:20c:29ff:fe76:6c14, length 24
-    4  15:15:02.391739 IP6 version error: 0 != 6
+    4  15:15:02.391739 IP6  [version 0 != 6] (invalid)