netlink: Lower netlink error report log level

This most likely meand, that the respective -diag module is missing.
In that case the caller would check it and _might_ not fail the dump
at once. Thus treat it as error.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

libnetlink.c

@@ -50,7 +50,7 @@ static int nlmsg_receive(char *buf, int len, int (*cb)(struct nlmsghdr *, void *
 			if (err->error == 0)
 				return 0;
 
-			pr_err("ERROR %d reported by netlink\n", err->error);
+			pr_warn("ERROR %d reported by netlink\n", err->error);
 			return err->error;
 		}
 		if (cb(hdr, arg))