loop: parse and store disk sequence number

When loop devices are re-used, the disk sequence number is increased.
Parse it when creating a loop device and store it.
The kernel will never return DISKSEQ=0, so use it to signal that it's
not supported by the current kernel.

src/basic/missing_loop.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <linux/fs.h>
 #include <linux/loop.h>
 
 #ifndef LOOP_CONFIGURE
@@ -13,3 +14,7 @@ struct loop_config {
 
 #define LOOP_CONFIGURE 0x4C0A
 #endif
+
+#ifndef BLKGETDISKSEQ
+#define BLKGETDISKSEQ _IOR(0x12,128,__u64)
+#endif