selftests: forwarding: Make VXLAN ECN encap tests more robust

These tests sometimes fail on the netdev CI because the expected number
of packets is larger than expected [1].

Make the tests more robust by specifically matching on VXLAN
encapsulated packets and allowing up to five stray packets instead of
just two.

[1]
[...]
# TEST: VXLAN: ECN encap: 0x00->0x00 [FAIL]
# v1: Expected to capture 10 packets, got 13.
# TEST: VXLAN: ECN encap: 0x01->0x01 [ OK ]
# TEST: VXLAN: ECN encap: 0x02->0x02 [ OK ]
# TEST: VXLAN: ECN encap: 0x03->0x02 [ OK ]
[...]

Signed-off-by: Ido Schimmel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/forwarding/vxlan_bridge_1d.sh

···
 	local delta=$((t1 - t0))
 
 	# Tolerate a couple stray extra packets.
-	((expect <= delta && delta <= expect + 2))
+	((expect <= delta && delta <= expect + 5))
 	check_err $? "$capture_dev: Expected to capture $expect packets, got $delta."
 }
 
···
 	RET=0
 
 	tc filter add dev v1 egress pref 77 prot ip \
-		flower ip_tos $tos action pass
+		flower ip_tos $tos ip_proto udp dst_port $VXPORT action pass
 	sleep 1
 	vxlan_ping_test $h1 192.0.2.3 "-Q $q" v1 egress 77 10
 	tc filter del dev v1 egress pref 77 prot ip

tools/testing/selftests/net/forwarding/vxlan_bridge_1d_ipv6.sh

···
 	local delta=$((t1 - t0))
 
 	# Tolerate a couple stray extra packets.
-	((expect <= delta && delta <= expect + 2))
+	((expect <= delta && delta <= expect + 5))
 	check_err $? "$capture_dev: Expected to capture $expect packets, got $delta."
 }
 
···
 	RET=0
 
 	tc filter add dev v1 egress pref 77 protocol ipv6 \
-		flower ip_tos $tos action pass
+		flower ip_tos $tos ip_proto udp dst_port $VXPORT action pass
 	sleep 1
 	vxlan_ping_test $h1 2001:db8:1::3 "-Q $q" v1 egress 77 10
 	tc filter del dev v1 egress pref 77 protocol ipv6