UPSTREAM: 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.

Bug: 254441685
Fixes: b673e24aad36 ("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>
(cherry picked from commit 782c72af567fc2ef09bd7615d0307f24de72c7e0)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I1eb2a391137f3b9415dc0826a52f5f27802331bc