scsi: target/iscsi: Detect conn_cmd_list corruption early

Certain behavior of an initiator can cause the target driver to send both a
reject and a SCSI response. If that happens two target_put_sess_cmd() calls
will occur without the command having been removed from conn_cmd_list. In
other words, conn_cmd_list will get corrupted once the freed memory is
reused. Although the Linux kernel can detect list corruption if list
debugging is enabled, in this case the context in which list corruption is
detected is not related to the context that caused list corruption.  Hence
add WARN_ON() statements that report the context that is causing list
corruption.

Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>