loop-util: rework how we lock loopback block devices

Let's rework how we lock loopback block devices in two ways:

1. Lock a separate fd, instead of the main block device fd. We already
   did that for our internal locking when allocating loopback block
   devices, but do so for the exposed locking (i.e.
   loop_device_flock()), too, so that the lock is independent of the
   main fd we actually use of IO.

2. Instead of locking the device during allocation of the loopback
   device, then unlocking it (which will make udev run), and then
   re-locking things if we need, let's instead just keep the lock the
   whole time, to make things a bit safer and faster, and not have to
   wait for udev at all. This is done by adding a "lock_op" parameter to
   loop device allocation functions that declares the initial state of
   the lock, and is one of LOCK_UN/LOCK_SH/LOCK_EX. This change also
   shortens a lot of code, since we allocate + immediately lock loopback
   devices pretty much everywhere.

src/shared/loop-util.h

@@ -10,6 +10,7 @@ typedef struct LoopDevice LoopDevice;
 
 struct LoopDevice {
         int fd;
+        int lock_fd;
         int nr;
         dev_t devno;
         char *node;
@@ -19,9 +20,9 @@ struct LoopDevice {
         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);
-int loop_device_make_by_path(const char *path, int open_flags, uint32_t loop_flags, LoopDevice **ret);
-int loop_device_open(const char *loop_path, int open_flags, LoopDevice **ret);
+int loop_device_make(int fd, int open_flags, uint64_t offset, uint64_t size, uint32_t loop_flags, int lock_op, LoopDevice **ret);
+int loop_device_make_by_path(const char *path, int open_flags, uint32_t loop_flags, int lock_op, LoopDevice **ret);
+int loop_device_open(const char *loop_path, int open_flags, int lock_op, LoopDevice **ret);
 
 LoopDevice* loop_device_unref(LoopDevice *d);
 DEFINE_TRIVIAL_CLEANUP_FUNC(LoopDevice*, loop_device_unref);