[IPV6]: Fix inet6_init missing unregister.

Based mostly upon a patch from Olaf Kirch <okir@suse.de>

When initialization fails in inet6_init(), we should
unregister the PF_INET6 socket ops.

Also, check sock_register()'s return value for errors.

Signed-off-by: David S. Miller <davem@davemloft.net>