loop-util: track CLOCK_MONOTONIC timestamp immediately before attaching a loopback device

This is similar to the preceding work to store the uevent seqnum, but
this stores the CLOCK_MONOTONIC timestamp.

Why? This allows to validate udev database entries, to determine if they
were created *after* we attached the device.

The uevent seqnum logic allows us to validate uevent, and the timestamp
database entries, hence together we should be able to validate both
sources of truth for us.

(note that this is all racy, just a bit less racy, since we cannot
atomically attach loopback devices and get the timestamp for it, the
same way we can't get the uevent seqnum. Thus is shortens the race
window, but doesn#t close it).

src/shared/loop-util.h

@@ -2,6 +2,7 @@
 #pragma once
 
 #include "macro.h"
+#include "time-util.h"
 
 typedef struct LoopDevice LoopDevice;
 
@@ -14,6 +15,7 @@ struct LoopDevice {
         char *node;
         bool relinquished;
         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 */
 };
 
 int loop_device_make(int fd, int open_flags, uint64_t offset, uint64_t size, uint32_t loop_flags, LoopDevice **ret);