selftests: forwarding: mirror_gre_vlan_bridge_1q: Fix roaming test

ARP or ND traffic can cause spurious migration of FDB back to $swp3.
Mirroring is then updated in accordance with the change, and mirrored
packets are seen at h3, causing a failure.

Detect the case of this spurious roaming, and retry the test.

Signed-off-by: Petr Machata <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/mirror_gre_vlan_bridge_1q.sh

···
 	mirror_install $swp1 ingress $tundev "matchall $tcflags"
 	quick_test_span_gre_dir $tundev ingress
 
-	bridge fdb del dev $swp3 $h3mac vlan 555 master
-	bridge fdb add dev $swp2 $h3mac vlan 555 master
-	sleep 1
-	fail_test_span_gre_dir $tundev ingress
+	while ((RET == 0)); do
+		bridge fdb del dev $swp3 $h3mac vlan 555 master 2>/dev/null
+		bridge fdb add dev $swp2 $h3mac vlan 555 master
+		sleep 1
+		fail_test_span_gre_dir $tundev ingress
 
-	bridge fdb del dev $swp2 $h3mac vlan 555 master
+		if ! bridge fdb sh dev $swp2 vlan 555 master \
+		    | grep -q $h3mac; then
+			printf "TEST: %-60s  [RETRY]\n" \
+				"$what: MAC roaming ($tcflags)"
+			# ARP or ND probably reprimed the FDB while the test
+			# was running. We would get a spurious failure.
+			RET=0
+			continue
+		fi
+		break
+	done
+
+	bridge fdb del dev $swp2 $h3mac vlan 555 master 2>/dev/null
 	# Re-prime FDB
 	$ARPING -I br1.555 192.0.2.130 -fqc 1
 	sleep 1