selftests: tc_actions: don't dump 2MB of \0 to stdout

Since we started running selftests in NIPA we have been seeing tc_actions.sh generate a soft lockup warning on ~20% of the runs. On the pre-netdev foundation setup it was actually a missed irq splat from the console. Now it's either that or a lockup.

I initially suspected a socket locking issue since the test is exercising local loopback with act_mirred.
After hours of staring at this I noticed in strace that ncat when -o $file is specified _both_ saves the output to the file and still prints it to stdout. Because the file being sent is constructed with:

dd conv=sparse status=none if=/dev/zero bs=1M count=2 of=$mirred

                                ^^^^^^^^^

the data printed is all \0. Most terminals don't display nul characters (and neither does vng output capture save them). But QEMU's serial console still has to poke them thru which is very slow and causes the lockup (if the file is >600kB).

Replace the '-o $file' with '> $file'. This speeds the test up from 2m20s to 18s on debug kernels, and prevents the warnings.

---
CC: [email protected]
CC: [email protected]
CC: [email protected]
CC: [email protected]
CC: [email protected]

---
 tools/testing/selftests/net/forwarding/tc_actions.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index ea89e558672d..86edbc7e2489 100755
@@ -223,7 +223,7 @@ mirred_egress_to_ingress_tcp_test()
 		ip_proto icmp \
 			action drop
 
-	ip vrf exec v$h1 ncat --recv-only -w10 -l -p 12345 -o $mirred_e2i_tf2 &
+	ip vrf exec v$h1 ncat --recv-only -w10 -l -p 12345 > $mirred_e2i_tf2 &
 	local rpid=$!
 	ip vrf exec v$h1 ncat -w1 --send-only 192.0.2.2 12345 <$mirred_e2i_tf1
 	wait -n $rpid

2.53.0

Ouch!

I exercise this by running ncat locally on Fedora 43.

$ ncat --version
Ncat: Version 7.92 ( https://nmap.org/ncat )

And confirmed the behaviour you describe.