platform: downgrade severity of message for failure to create NMPNetns

Under restricted permissions (like inside a podman container) opening
"/proc/self/ns/net" fails with Permission denied. Consequently we cannot
create our bottom NMPNetns instance. That is mostly fine, however we
would log an error message with severity <error>.

Note that test "src/core/platform/tests/test-platform-general" asserts
that no <warn> and <error> messages get logged. Hence, the test will
fail.

That is undesirable. Downgrade the message to <debug> so that the test
passes. Also, it's not clear that this error message is useful here.
Being unable to open a netns fd is fine and not necessarily an error
condition.

(cherry picked from commit 0213300dce)

shared/nm-platform/nmp-netns.c

@@ -171,7 +171,7 @@ _netns_stack_get_impl(void)
     netns = _netns_new(&error);
 
     if (!netns) {
-        _LOGE(NULL, "failed to create initial netns: %s", error->message);
+        _LOGD(NULL, "failed to create initial netns: %s", error->message);
         return s;
     }