flow: fix incorrect padding length checking in ipv6_sanity_check

The padding length is (packet size - ipv6 header length - ipv6 plen).  This
patch fixes incorrect padding size checking in ipv6_sanity_check.

Acked-by: William Tu <u9012063@gmail.com>
Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/flow.c

@@ -699,7 +699,7 @@ ipv6_sanity_check(const struct ovs_16aligned_ip6_hdr *nh, size_t size)
         return false;
     }
     /* Jumbo Payload option not supported yet. */
-    if (OVS_UNLIKELY(size - plen > UINT8_MAX)) {
+    if (OVS_UNLIKELY(size - (plen + IPV6_HEADER_LEN) > UINT8_MAX)) {
         return false;
     }