IPv6/Multi: do the ICMP checksum calculations in the same way as for TCP and UDP.

[htibosch]

Description

Some change to ICMP/ping handling:

• Depending on ipconfigFORCE_IP_DONT_FRAGMENT, set or clear the fragment field usFragmentOffset.
• Depending on ipconfigDRIVER_INCLUDED_TX_IP_CHECKSUM, do a complete checksum calculation, or just clear the ICMP protocol checksum.
• Check the return value of prvChecksumICMPv6Checks():

-    prvChecksumICMPv6Checks( uxBufferLength, pxSet );
+    xReturn = prvChecksumICMPv6Checks( uxBufferLength, pxSet );

Test Steps

I tested while setting:

#define ipconfigDRIVER_INCLUDED_RX_IP_CHECKSUM   0
#define ipconfigDRIVER_INCLUDED_TX_IP_CHECKSUM   0

meaning that the IP-task will calculate all checksums "manually".

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