selftests: netfilter: fix conntrack_dump_flush retval on unsupported kernel

With CONFIG_NETFILTER=n test passes instead of skip.  Before:

 ./run_kselftest.sh -t net/netfilter:conntrack_dump_flush
[..]
 # Starting 3 tests from 1 test cases.
 #  RUN           conntrack_dump_flush.test_dump_by_zone ...
 mnl_socket_open: Protocol not supported
[..]
 ok 3 conntrack_dump_flush.test_flush_by_zone_default
 # PASSED: 3 / 3 tests passed.
 # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0

After:
  mnl_socket_open: Protocol not supported
[..]
  ok 3 conntrack_dump_flush.test_flush_by_zone_default # SKIP cannot open netlink_netfilter socket
  # PASSED: 3 / 3 tests passed.
  # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:3 error:0
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240422103358.3511-1-fw@strlen.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/netfilter/conntrack_dump_flush.c

@@ -313,13 +313,11 @@ FIXTURE_SETUP(conntrack_dump_flush)
 	self->sock = mnl_socket_open(NETLINK_NETFILTER);
 	if (!self->sock) {
 		perror("mnl_socket_open");
-		exit(EXIT_FAILURE);
+		SKIP(return, "cannot open netlink_netfilter socket");
 	}
 
-	if (mnl_socket_bind(self->sock, 0, MNL_SOCKET_AUTOPID) < 0) {
-		perror("mnl_socket_bind");
-		exit(EXIT_FAILURE);
-	}
+	ret = mnl_socket_bind(self->sock, 0, MNL_SOCKET_AUTOPID);
+	EXPECT_EQ(ret, 0);
 
 	ret = conntracK_count_zone(self->sock, TEST_ZONE_ID);
 	if (ret < 0 && errno == EPERM)