ublk: check list membership before cancelling batch fetch command

ublk: check list membership before cancelling batch fetch command

Add !list_empty(&fcmd->node) check in ublk_batch_cancel_cmd() to ensure
the fcmd hasn't already been removed from the list. Once an fcmd is
removed from the list, it's considered claimed by whoever removed it
and will be freed by that path.

Meantime switch to list_del_init() for deleting it from list.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>