selftests: pmtu: detect correct binary to ping ipv6 addresses

Some systems don't have the ping6 binary anymore, and use ping for
everything. Detect the absence of ping6 and try to use ping instead.

Fixes: d1f1b9cb ("selftests: net: Introduce first PMTU test")
Signed-off-by: NSabrina Dubroca <sd@queasysnail.net>
Acked-by: NStefano Brivio <sbrivio@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/pmtu.sh

@@ -46,6 +46,9 @@
 # Kselftest framework requirement - SKIP code is 4.
 ksft_skip=4
 
+# Some systems don't have a ping6 binary anymore
+which ping6 > /dev/null 2>&1 && ping6=$(which ping6) || ping6=$(which ping)
+
 tests="
 	pmtu_vti6_exception		vti6: PMTU exceptions
 	pmtu_vti4_exception		vti4: PMTU exceptions
@@ -274,7 +277,7 @@ test_pmtu_vti6_exception() {
 	mtu "${ns_b}" veth_b 4000
 	mtu "${ns_a}" vti6_a 5000
 	mtu "${ns_b}" vti6_b 5000
-	${ns_a} ping6 -q -i 0.1 -w 2 -s 60000 ${vti6_b_addr} > /dev/null
+	${ns_a} ${ping6} -q -i 0.1 -w 2 -s 60000 ${vti6_b_addr} > /dev/null
 
 	# Check that exception was created
 	if [ "$(route_get_dst_pmtu_from_exception "${ns_a}" ${vti6_b_addr})" = "" ]; then