selftests/net: specify the interface when do arping

When do arping, the interface need to be specified. Or we will
get error: Interface "lo" is not ARPable. And the test failed.
]# ./arp_ndisc_untracked_subnets.sh
    TEST: test_arp:  accept_arp=0                                       [ OK ]
    TEST: test_arp:  accept_arp=1                                       [FAIL]
    TEST: test_arp:  accept_arp=2  same_subnet=0                        [ OK ]
    TEST: test_arp:  accept_arp=2  same_subnet=1                        [FAIL]

After fix:
]# ./arp_ndisc_untracked_subnets.sh
    TEST: test_arp:  accept_arp=0                                       [ OK ]
    TEST: test_arp:  accept_arp=1                                       [ OK ]
    TEST: test_arp:  accept_arp=2  same_subnet=0                        [ OK ]
    TEST: test_arp:  accept_arp=2  same_subnet=1                        [ OK ]

Fixes: 0ea7b0a4 ("selftests: net: arp_ndisc_untracked_subnets: test for arp_accept and accept_untracked_na")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/arp_ndisc_untracked_subnets.sh

@@ -150,7 +150,7 @@ arp_test_gratuitous() {
 	fi
 	# Supply arp_accept option to set up which sets it in sysctl
 	setup ${arp_accept}
-	ip netns exec ${HOST_NS} arping -A -U ${HOST_ADDR} -c1 2>&1 >/dev/null
+	ip netns exec ${HOST_NS} arping -A -I ${HOST_INTF} -U ${HOST_ADDR} -c1 2>&1 >/dev/null
 
 	if verify_arp $1 $2; then
 		printf "    TEST: %-60s  [ OK ]\n" "${test_msg[*]}"