target/iscsi: Detect conn_cmd_list corruption early

Certain behavior of the 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.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>