init_tgt isn't always necessary

[mattysweeps]

__ublksrv_tgt_init which is invoked from ublksrv_dev_init checks:

if (!ops->init_tgt)
	return -EINVAL;

In my use case I don't have any work to do here, so I define an empty function:

static int init_tgt(struct ublksrv_dev *dev, int type, int argc, char *argv[]) {
  return 0;
}

static struct ublksrv_tgt_type ops = {
...
    .init_tgt = init_tgt,
...
};

This works fine, but I'm curious if the requirement for init_tgt can be removed. The reason why I don't have anything to do here is I'm using ublksrv as a library (not integrating a new type into the ublk cli).

I thought I'd open this issue to learn more about why init_tgt was required. Thank you,

[ming1]

->init_tgt() is usually useful for initializing everything for setting up the target,
but it isn't a must.

I have relaxed the limit by commit cbe556e.

Thanks,