iscsi: Kick threads out of iscsi_ioctl() during unload.

iscsid can be sleeping in iscsi_ioctl() causing the destroy_dev() to
sleep forever if iscsi.ko is unloaded while iscsid is running.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit 89df484739efe93b52da467f35255ae538bb946b)