target/iblock: Use request_queue->nr_request for se_device defaults

This patch converts iblock_create_virtdevice() to use request_queue->nr_request
for se_dev_limits usage of ->hw_queue_depth and ->queue_depth for individual
struct se_device export.

It also removes the now unused defines for IBLOCK_DEVICE_QUEUE_DEPTH and
IBLOCK_MAX_DEVICE_QUEUE_DEPTH

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>