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

net/ipv4/af_inet.c

@@ -1043,22 +1043,16 @@ void inet_register_protosw(struct inet_protosw *p)
 		goto out_illegal;
 
 	/* If we are trying to override a permanent protocol, bail. */
-	answer = NULL;
 	last_perm = &inetsw[p->type];
 	list_for_each(lh, &inetsw[p->type]) {
 		answer = list_entry(lh, struct inet_protosw, list);
-
 		/* Check only the non-wild match. */
-		if (INET_PROTOSW_PERMANENT & answer->flags) {
-			if (protocol == answer->protocol)
-				break;
-			last_perm = lh;
-		}
-
-		answer = NULL;
+		if ((INET_PROTOSW_PERMANENT & answer->flags) == 0)
+			break;
+		if (protocol == answer->protocol)
+			goto out_permanent;
+		last_perm = lh;
 	}
-	if (answer)
-		goto out_permanent;
 
 	/* Add the new entry after the last permanent entry if any, so that
 	 * the new entry does not override a permanent entry when matched with