loop-util: read kernel's uevent seqnum right before attaching a loopback device

Later, this will allow us to ignore uevents from earlier attachments a
bit better, as we can compare uevent seqnums with this boundary. It's
not a full fix for the race though, since we cannot atomically determine
the uevent and attach the device, but it at least shortens the window a
bit.

src/shared/loop-util.h

@@ -13,6 +13,7 @@ struct LoopDevice {
         dev_t devno;
         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 */
 };
 
 int loop_device_make(int fd, int open_flags, uint64_t offset, uint64_t size, uint32_t loop_flags, LoopDevice **ret);