loop-util: use new LOOP_CONFIGURE ioctl

LOOP_CONFIGURE allows us to configure a loopback device in one ioctl
instead of two, which is not just faster but also removes the race that
udev might start probing the device before we adjusted things properly.

Unfortunately LOOP_CONFIGURE is broken in regards to LO_FLAGS_PARTSCAN
as of kernel 5.8.0. This patch contains a work-around for that, to
fallback to old behaviour if partition scanning is requested but does
not work. Sucks a bit.

Proposed upstream fix for that issue:

https://lkml.org/lkml/2020/8/6/97

src/basic/missing_loop.h

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <linux/loop.h>
+
+#ifndef LOOP_CONFIGURE
+struct loop_config {
+        __u32 fd;
+        __u32 block_size;
+        struct loop_info64 info;
+        __u64 __reserved[8];
+};
+
+#define LOOP_CONFIGURE 0x4C0A
+#endif