Add session locking in cfiscsi_ioctl_handoff().

While there, remove ifdef around cs_target check in cfiscsi_ioctl_list().
I am not sure why this ifdef was added, but without this check code will
crash below on NULL dereference.

Submitted by: Aleksandr Fedorov <aleksandr.fedorov@itglobal.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24587