iscsi: Fix the issue caused by cleanup LUNs when exiting connection

When exiting connection, all active tasks are aborted by
spdk_iscsi_tgt_node_cleanup() in lib/iscsi/tgt_node.c.

In spdk_iscsi_tgt_node_cleanup(), spdk_iscsi_op_abort_task_set()
is called by using a temporary task without associating PDU.

This call of spdk_iscsi_op_abort_task_set() have resulted in
assertion in spdk_iscsi_conn_abort_queued_datain_tasks() because
task->pdu is NULL.

The fix is to remove the assertion in
spdk_iscsi_conn_abort_queued_datain_tasks() and add check if
pdu == NULL as true condition.

This issue was detected by ip_migration test. ip_migration test
runs only in nightly tests now but it consumes only 26 seconds,
and the next patch moves ip_migration test to per-patch tests.

Fixes #544

Change-Id: I7719ad57d22c4354530adabfcdc5a818c804b841
Signed-off-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-on: https://review.gerrithub.io/438223


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>