[IPV6]: Remove circular dependency on if_inet6.h

net/if_inet6.h includes linux/ipv6.h which also tries to include
net/if_inet6.h.  Since the latter only needs it for forward
declarations, we can fix this by adding the declarations.

A number of files are implicitly including net/if_inet6.h through
linux/ipv6.h.  They also use net/ipv6.h so this patch includes
net/if_inet6.h there.

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