selftests: netfilter: Fix skip of wildcard interface test

The script is supposed to skip wildcard interface testing if unsupported
by the host's nft tool. The failing check caused script abort due to
'set -e' though. Fix this by running the potentially failing nft command
inside the if-conditional pipe.

Fixes: 73db1b5 ("selftests: netfilter: Torture nftables netdev hooks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/20250527094117.18589-1-phil@nwl.cc
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Phil Sutter

tools/testing/selftests/net/netfilter/nft_interface_stress.sh

@@ -97,15 +97,18 @@ kill $nft_monitor_pid
 kill $rename_loop_pid
 wait
 
-ip netns exec $nsr nft -f - <<EOF
+wildcard_prep() {
+	ip netns exec $nsr nft -f - <<EOF
 table ip t {
 	flowtable ft_wild {
 		hook ingress priority 0
 		devices = { wild* }
 	}
 }
 EOF
-if [[ $? -ne 0 ]]; then
+}
+
+if ! wildcard_prep; then
 	echo "SKIP wildcard tests: not supported by host's nft?"
 else
 	for ((i = 0; i < 100; i++)); do