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: 6f84090333bb ("selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest")
Signed-off-by: Hangbin Liu <[e-mail]>
Acked-by: Nikolay Aleksandrov <[e-mail]>
Reviewed-by: Johannes Nixdorf <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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

···
 	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
···
 		check_limit_one_type $type_args
 	done
 }
+
+check_fdb_n_learned_support
 
 trap cleanup EXIT