net: only check perm protocol when register proto

The permanent protocol nodes are at the head of the list,
So only need check all these nodes.

No matter the new node is permanent or not,
insert the new node after the last permanent protocol node,

If the new node conflicts with existing permanent node,
return error.

Signed-off-by: Martin Zhang <martinbj2008@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>