net/ipv6/raw.c (icmpv6_filter): try to fetch only the minimum ICMPv6 header

The kernel assumes that any valid ICMPv6 message is at least eight
bytes long. RFC 4443 seems to allow even a zero-sized body, making
the minimum allowable size four bytes.

The DIS message of RPL (RFC 6550) is something that exists in real
life, has a minimum length of six bytes, and is thus blocked by
icmpv6_filter.

A similar restriction exists in net/ipv6/icmp.c (icmpv6_rcv) but
causes no harm there since all potential recipients at that point
expect a larger payload.