inet: Remove bogus IGMPv3 report handling

Currently we treat IGMPv3 reports as if it were an IGMPv2/v1 report.
This is broken as IGMPv3 reports are formatted differently.  So we
end up suppressing a bogus multicast group (which should be harmless
as long as the leading reserved field is zero).

In fact, IGMPv3 does not allow membership report suppression so
we should simply ignore IGMPv3 membership reports as a host.

This patch does exactly that.  I kept the case statement for it
so people won't accidentally add it back thinking that we overlooked
this case.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>