iscsi: Remove icl_soft-only fields from struct icl_conn.

Create a struct icl_soft_conn which extends struct icl_conn and
move fields only used by icl_soft from struct icl_conn to
struct icl_soft_conn.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D31414

(cherry picked from commit 87322a907545fa76fbaf7949f80e85b1377a53ad)