mwan3: support latest iputils ping

iputils upstream changed build params with version s20200821
Latest OpenWRT iputils ping now appears to report the openwrt
version tag, rather than iputils date tag

This commit sends a test ping to localhost to evaluate the
capabilities of iputils ping.

Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>

net/mwan3/files/usr/sbin/mwan3track

@@ -40,19 +40,35 @@ if_up() {
 	stop_subprocs
 }
 
+ping_test_host() {
+	if [ "$FAMILY" = "ipv6" ]; then
+		echo "::1"
+	else
+		echo "127.0.0.1"
+	fi
+}
+
+get_ping_command() {
+	if [ -x "/usr/bin/ping" ] && /usr/bin/ping -${FAMILY#ipv} -c1 -q $(ping_test_host) &>/dev/null; then
+		# -4 option added in iputils c3e68ac6 so need to check if we can use it
+		# or if we must use ping and ping6
+		echo "/usr/bin/ping -${FAMILY#ipv}"
+	elif [ "$FAMILY" = "ipv6" ] && [ -x "/usr/bin/ping6" ]; then
+		echo "/usr/bin/ping6"
+	elif [ "$FAMILY" = "ipv4" ] && [ -x "/usr/bin/ping" ]; then
+		echo "/usr/bin/ping"
+	elif [ -x "/bin/ping" ]; then
+		echo "/bin/ping -${FAMILY#ipv}"
+	else
+		return 1
+	fi
+}
+
 validate_track_method() {
 	case "$1" in
 		ping)
-			if [ -x "/usr/bin/ping" ] && [ "$(/usr/bin/ping -V | grep -o '[0-9]*$')" -gt 20150519 ]; then
-				# -4 option added in iputils c3e68ac6
-				PING="/usr/bin/ping -${FAMILY#ipv}"
-			elif [ "$FAMILY" = "ipv6" ] && [ -x "/usr/bin/ping6" ]; then
-				PING="/usr/bin/ping6"
-			elif [ "$FAMILY" = "ipv4" ] && [ -x "/usr/bin/ping" ]; then
-				PING="/usr/bin/ping"
-			elif [ -x "/bin/ping" ]; then
-				PING="/bin/ping -${FAMILY#ipv}"
-			else
+			PING=$(get_ping_command)
+			if [ $? -ne 0 ]; then
 				LOG warn "Missing ping. Please enable BUSYBOX_DEFAULT_PING and recompile busybox or install iputils-ping package."
 				return 1
 			fi