selftests: netfilter: Fix and review rpath.sh

Address a few problems with the initial test script version:

* On systems with ip6tables but no ip6tables-legacy, testing for
ip6tables was disabled by accident.
* Firewall setup phase did not respect possibly unavailable tools.
* Consistently call nft via '$nft'.

Fixes: 6e31ce831c63b ("selftests: netfilter: Test reverse path filtering")
Signed-off-by: Phil Sutter <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

tools/testing/selftests/netfilter/rpath.sh

···
 
 if ip6tables-legacy --version >/dev/null 2>&1; then
 	ip6tables='ip6tables-legacy'
-elif ! ip6tables --version >/dev/null 2>&1; then
+elif ip6tables --version >/dev/null 2>&1; then
 	ip6tables='ip6tables'
 else
 	ip6tables=''
···
 ip -net "$ns2" a a fec0:42::1/64 dev d0 nodad
 
 # firewall matches to test
-ip netns exec "$ns2" "$iptables" -t raw -A PREROUTING -s 192.168.0.0/16 -m rpfilter
-ip netns exec "$ns2" "$ip6tables" -t raw -A PREROUTING -s fec0::/16 -m rpfilter
-ip netns exec "$ns2" nft -f - <<EOF
+[ -n "$iptables" ] && ip netns exec "$ns2" \
+	"$iptables" -t raw -A PREROUTING -s 192.168.0.0/16 -m rpfilter
+[ -n "$ip6tables" ] && ip netns exec "$ns2" \
+	"$ip6tables" -t raw -A PREROUTING -s fec0::/16 -m rpfilter
+[ -n "$nft" ] && ip netns exec "$ns2" $nft -f - <<EOF
 table inet t {
 	chain c {
 		type filter hook prerouting priority raw;
···
 	if [ -n "$nft" ]; then
 		(
 			echo "delete table inet t";
-			ip netns exec "$ns2" nft -s list table inet t;
-		) | ip netns exec "$ns2" nft -f -
+			ip netns exec "$ns2" $nft -s list table inet t;
+		) | ip netns exec "$ns2" $nft -f -
 	fi
 
 	# test 1: martian traffic should fail rpfilter matches