icmp: add helpers to recognize ICMP error packets

Add two helper functions, one for IPv4 and one for IPv6, to recognize
the ICMP packets which are error responses.
This packets are special because they have as payload the original
header of the packet which generated it (RFC 792 says at least 8 bytes,
but Linux actually includes much more than that).

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>