Fix iSCSI target crash on session reinstation.

The crash scenario goes like this: there's a thread waiting on "reinstate";
because it doesn't update the timeout counter it gets terminated by the
callout; at this point the maintenance thread starts the termination routine.
The first thread finishes waiting, proceeds to icl_conn_handoff(), and drops
the refcount, which allows the maintenance thread to free its resources. At
this point another thread receives a PDU. Boom.

PR: 222898, 219866
Reported by: Eugene M. Zheganin <emz at norma.perm.ru>
Tested by: Eugene M. Zheganin <emz at norma.perm.ru>
Reviewed by: mav@ (earlier version)
MFC after: 2 weeks
Sponsored by: playkey.net