[IPV6] addrconf: Fix IPv6 on tuntap tunnels

The recent patch that added ipv6_hwtype is broken on tuntap tunnels.
Indeed, it's broken on any device that does not pass the ipv6_hwtype
test.

The reason is that the original test only applies to autoconfiguration,
not IPv6 support.  IPv6 support is allowed on any device.  In fact,
even with the ipv6_hwtype patch applied you can still add IPv6 addresses
to any interface that doesn't pass thw ipv6_hwtype test provided that
they have a sufficiently large MTU.  This is a serious problem because
come deregistration time these devices won't be cleaned up properly.

I've gone back and looked at the rationale for the patch.  It appears
that the real problem is that we were creating IPv6 devices even if the
MTU was too small.  So here's a patch which fixes that and reverts the
ipv6_hwtype stuff.

Thanks to Kanru Chen for reporting this issue.

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