Add a simple check that data written through the loop device is
actually ending up in the file. NB: this this will _fail_ if the
fd is cleared via `clear_fd` without the use of `flush_buf`. It
seems that the kernel (as of 5.13.8) will indeed not clear the
buffer cache of the loop device if the backing file is detached
via `LOOP_CLR_FD`. On the other hand, if the autoclear flag is,
i.e. the backing file cleared when the last file descriptor of
the loop device is closed, the buffer cached will be cleared as
part of the `release` operation of the block device.
4126a3af7c