driver-core: skip uevent generation when nobody is listening

Most network namespaces unlikely have listeners to uevents, and should
benefit from skipping all the string copies.

Cc: Eric Biederman <[e-mail]>
Signed-off-by: Kay Sievers <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

lib/kobject_uevent.c

···
 		struct sk_buff *skb;
 		size_t len;
 
+		if (!netlink_has_listeners(uevent_sock, 1))
+			continue;
+
 		/* allocate message with the maximum possible size */
 		len = strlen(action_string) + strlen(devpath) + 2;
 		skb = alloc_skb(len + env->buflen, GFP_KERNEL);