selftests/landlock: Fix format warning for __u64 in net_test

On architectures where __u64 is unsigned long (e.g. powerpc64), using
%llx to format a __u64 triggers a -Wformat warning because %llx expects
unsigned long long.  Cast the argument to unsigned long long.

Cc: Günther Noack <gnoack@google.com>
Cc: stable@vger.kernel.org
Fixes: a549d055a2 ("selftests/landlock: Add network tests")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202604020206.62zgOTeP-lkp@intel.com/
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260402192608.1458252-6-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/net_test.c

@@ -1356,7 +1356,7 @@ TEST_F(mini, network_access_rights)
 					    &net_port, 0))
 		{
 			TH_LOG("Failed to add rule with access 0x%llx: %s",
-			       access, strerror(errno));
+			       (unsigned long long)access, strerror(errno));
 		}
 	}
 	EXPECT_EQ(0, close(ruleset_fd));