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/shared/loop-util.h

@@ -14,6 +14,7 @@ struct LoopDevice {
         dev_t devno;
         char *node;
         bool relinquished;
+        uint64_t diskseq; /* Block device sequence number, monothonically incremented by the kernel on create/attach, or 0 if we don't know */
         uint64_t uevent_seqnum_not_before; /* uevent sequm right before we attached the loopback device, or UINT64_MAX if we don't know */
         usec_t timestamp_not_before; /* CLOCK_MONOTONIC timestamp taken immediately before attaching the loopback device, or USEC_INFINITY if we don't know */
 };