selftests: forwarding: skip if kernel not support setting bridge fdb learning limit

If the testing kernel doesn't support setting fdb_max_learned or show
fdb_n_learned, just skip it. Or we will get errors like

./bridge_fdb_learning_limit.sh: line 218: [: null: integer expression expected
./bridge_fdb_learning_limit.sh: line 225: [: null: integer expression expected

Fixes: 6f840903 ("selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Johannes Nixdorf <jnixdorf-oss@avm.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -178,6 +178,22 @@ fdb_del()
 	check_err $? "Failed to remove a FDB entry of type ${type}"
 }
 
+check_fdb_n_learned_support()
+{
+	if ! ip link help bridge 2>&1 | grep -q "fdb_max_learned"; then
+		echo "SKIP: iproute2 too old, missing bridge max learned support"
+		exit $ksft_skip
+	fi
+
+	ip link add dev br0 type bridge
+	local learned=$(fdb_get_n_learned)
+	ip link del dev br0
+	if [ "$learned" == "null" ]; then
+		echo "SKIP: kernel too old; bridge fdb_n_learned feature not supported."
+		exit $ksft_skip
+	fi
+}
+
 check_accounting_one_type()
 {
 	local type=$1 is_counted=$2 overrides_learned=$3
@@ -274,6 +290,8 @@ check_limit()
 	done
 }
 
+check_fdb_n_learned_support
+
 trap cleanup EXIT
 
 setup_prepare