IPv6: add "disable" module parameter support to ipv6.ko

Add "disable" module parameter support to ipv6.ko by specifying
"disable=1" on module load.  We just do the minimum of initializing
inetsw6[] so calls from other modules to inet6_register_protosw()
won't OOPs, then bail out.  No IPv6 addresses or sockets can be
created as a result, and a reboot is required to enable IPv6.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>