Fix MAX_PKT{4,6}_SZ to account for icmpEchoData padding (#887)

Fix MAX_PKT{4,6}_SZ to account for icmpEchoData padding (#887)

The bug was exposed by GCC v11 on Ubuntu Impish:

    Icmp4.cc:116:11: error: array subscript icmpEchoData[0] is partly
        outside array bounds of char[282] [-Werror=array-bounds]
        echo->opcode = (unsigned char) opcode;

The array the compiler is talking about is the pkt buffer. That buffer
size (i.e. MAX_PKT4_SZ) was calculated under the faulty assumption that
a compiler cannot add padding after icmphdr (when doing "icmp+1") and/or
between icmpEchoData data members. When compiler padded, the old
MAX_PKT4_SZ math stopped working.

Same for ICMPv6.