ipv4: Restore accept_local behaviour in fib_validate_source()

Commit 7a9bc9b81a5b ("ipv4: Elide fib_validate_source() completely when possible.")
introduced a short-circuit to avoid calling fib_validate_source when not
needed. That change took rp_filter into account, but not accept_local.
This resulted in a change of behaviour: with rp_filter and accept_local
off, incoming packets with a local address in the source field should be
dropped.

Here is how to reproduce the change pre/post 7a9bc9b81a5b commit:
-configure the same IPv4 address on hosts A and B.
-try to send an ARP request from B to A.
-The ARP request will be dropped before that commit, but accepted and answered
after that commit.

This adds a check for ACCEPT_LOCAL, to maintain full
fib validation in case it is 0. We also leave __fib_validate_source() earlier
when possible, based on the same check as fib_validate_source(), once the
accept_local stuff is verified.

Cc: Gregory Detal <[e-mail]>
Cc: Christoph Paasch <[e-mail]>
Cc: Hannes Frederic Sowa <[e-mail]>
Cc: Sergei Shtylyov <[e-mail]>
Signed-off-by: Sébastien Barré <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/fib_frontend.c

···
 				 u8 tos, int oif, struct net_device *dev,
 				 int rpf, struct in_device *idev, u32 *itag)
 {
-	int ret, no_addr, accept_local;
+	int ret, no_addr;
 	struct fib_result res;
 	struct flowi4 fl4;
 	struct net *net;
···
 
 	no_addr = idev->ifa_list == NULL;
 
-	accept_local = IN_DEV_ACCEPT_LOCAL(idev);
 	fl4.flowi4_mark = IN_DEV_SRC_VMARK(idev) ? skb->mark : 0;
 
 	net = dev_net(dev);
 	if (fib_lookup(net, &fl4, &res))
 		goto last_resort;
-	if (res.type != RTN_UNICAST) {
-		if (res.type != RTN_LOCAL || !accept_local)
-			goto e_inval;
-	}
+	if (res.type != RTN_UNICAST &&
+	    (res.type != RTN_LOCAL || !IN_DEV_ACCEPT_LOCAL(idev)))
+		goto e_inval;
+	if (!rpf && !fib_num_tclassid_users(dev_net(dev)) &&
+	    (dev->ifindex != oif || !IN_DEV_TX_REDIRECTS(idev)))
+		goto last_resort;
 	fib_combine_itag(itag, &res);
 	dev_match = false;
 
···
 	int r = secpath_exists(skb) ? 0 : IN_DEV_RPFILTER(idev);
 
 	if (!r && !fib_num_tclassid_users(dev_net(dev)) &&
+	    IN_DEV_ACCEPT_LOCAL(idev) &&
 	    (dev->ifindex != oif || !IN_DEV_TX_REDIRECTS(idev))) {
 		*itag = 0;
 		return 0;