wireguard: selftests: actually test for routing loops

We previously removed the restriction on looping to self, and then added
a test to make sure the kernel didn't blow up during a routing loop. The
kernel didn't blow up, thankfully, but on certain architectures where
skb fragmentation is easier, such as ppc64, the skbs weren't actually
being discarded after a few rounds through. But the test wasn't catching
this. So actually test explicitly for massive increases in tx to see if
we have a routing loop. Note that the actual loop problem will need to
be addressed in a different commit.

Fixes: b673e24aad ("wireguard: socket: remove errant restriction on looping to self")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/wireguard/netns.sh

@@ -276,7 +276,11 @@ n0 ping -W 1 -c 1 192.168.241.2
 n1 wg set wg0 peer "$pub2" endpoint 192.168.241.2:7
 ip2 link del wg0
 ip2 link del wg1
-! n0 ping -W 1 -c 10 -f 192.168.241.2 || false # Should not crash kernel
+read _ _ tx_bytes_before < <(n0 wg show wg1 transfer)
+! n0 ping -W 1 -c 10 -f 192.168.241.2 || false
+sleep 1
+read _ _ tx_bytes_after < <(n0 wg show wg1 transfer)
+(( tx_bytes_after - tx_bytes_before < 70000 ))
 
 ip0 link del wg1
 ip1 link del wg0