selftests: forwarding: ethtool_mm: Skip when MAC Merge is not supported

MAC Merge cannot be tested with veth pairs, resulting in failures:

 # ./ethtool_mm.sh
 [...]
 TEST: Manual configuration with verification: swp1 to swp2          [FAIL]
         Verification did not succeed

Fix by skipping the test when the interfaces do not support MAC Merge.

Fixes: e6991384 ("selftests: forwarding: add a test for MAC Merge layer")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-11-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -258,11 +258,6 @@ h2_destroy()
 
 setup_prepare()
 {
-	check_ethtool_mm_support
-	check_tc_fp_support
-	require_command lldptool
-	bail_on_lldpad "autoconfigure the MAC Merge layer" "configure it manually"
-
 	h1=${NETIFS[p1]}
 	h2=${NETIFS[p2]}
 
@@ -278,6 +273,19 @@ cleanup()
 	h1_destroy
 }
 
+check_ethtool_mm_support
+check_tc_fp_support
+require_command lldptool
+bail_on_lldpad "autoconfigure the MAC Merge layer" "configure it manually"
+
+for netif in ${NETIFS[@]}; do
+	ethtool --show-mm $netif 2>&1 &> /dev/null
+	if [[ $? -ne 0 ]]; then
+		echo "SKIP: $netif does not support MAC Merge"
+		exit $ksft_skip
+	fi
+done
+
 trap cleanup EXIT
 
 setup_prepare