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
Reviewed by: mav
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29688