loop-util: explicitly close loopback block device before sleeping

attach_empty() file takes a BSD file lock on the device, and we really
should release that before going to sleep. hence explicitly close the
block device before the sleep instead of relying on _cleanup_ to close
it after the sleep.

src/shared/loop-util.c

@@ -561,6 +561,9 @@ static int loop_device_make_internal(
                 if (++n_attempts >= 64) /* Give up eventually */
                         return -EBUSY;
 
+                /* Now close the loop device explicitly. This will release any lock acquired by
+                 * attach_empty_file() or similar, while we sleep below. */
+                loop = safe_close(loop);
                 loopdev = mfree(loopdev);
 
                 /* Wait some random time, to make collision less likely. Let's pick a random time in the