ipv6: Fix BUG when disabled ipv6 module is unloaded

Do not try to "uninitialize" ipv6 if its initialization had been skipped
because module parameter disable=1 had been specified.

Reported-by: Thomas Backlund <tmb@mandriva.org>
Signed-off-by: John Dykstra <john.dykstra1@gmail.com>
Acked-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>