linux-platform: fix a file descriptor leak

Coverity: CID 59384 (#3-1 of 3): Resource leak (RESOURCE_LEAK)26.
leaked_handle: Handle variable fd going out of scope leaks the handle

src/platform/nm-linux-platform.c

@@ -4827,6 +4827,7 @@ tun_add (NMPlatform *platform, const char *name, gboolean tap,
 	if (out_link)
 		*out_link = obj ? &obj->link : NULL;
 
+	close (fd);
 	return !!obj;
 }