net: checksum: Skip fragmented IP packets

To calculate the TCP/UDP checksum we need the whole datagram. Unless
the hardware has some logic to collect all fragments before sending
the whole datagram first, it can only be done by the network stack,
which is normally the case for the NICs we have seen so far.

Skip these fragmented IP packets to avoid checksum corruption.

Signed-off-by: Markus Carlstedt <markus.carlstedt@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/checksum.c

@@ -106,6 +106,10 @@ void net_checksum_calculate(uint8_t *data, int length)
         return; /* not IPv4 */
     }
 
+    if (IP4_IS_FRAGMENT(ip)) {
+        return; /* a fragmented IP packet */
+    }
+
     ip_len = lduw_be_p(&ip->ip_len);
 
     /* Last, check that we have enough data for the all IP frame */