target: Don't force session reset if queue_depth does not change

Keeping in the idempotent nature of target_core_fabric_configfs.c,
if a queue_depth value is set and it's the same as the existing
value, don't attempt to force session reinstatement.

Reported-by: Raghu Krishnamurthy <rk@datera.io>
Cc: Raghu Krishnamurthy <rk@datera.io>
Tested-by: Gary Guo <ghg@datera.io>
Cc: Gary Guo <ghg@datera.io>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>