icmpv6_filter: allow ICMPv6 messages with bodies < 4 bytes

By using sizeof(_hdr), net/ipv6/raw.c:icmpv6_filter implicitly assumes
that any valid ICMPv6 message is at least eight bytes long, i.e., that
the message body is at least four bytes.

The DIS message of RPL (RFC 6550 section 6.2, from the 6LoWPAN world),
has a minimum length of only six bytes, and is thus blocked by
icmpv6_filter.

RFC 4443 seems to allow even a zero-sized body, making the minimum
allowable message size four bytes.

Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>