[NET]: NPROTO is redundant; it's equal to AF_MAX/PF_MAX.

DaveM pointed out NPROTO exposed to userspace, so keep it around,
just make sure it stays in sync.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>