Close race on req->wrk clearing introduced in 6f2e1bcd

The disembarking thread would clear the req->wrk pointer in
CNT_Request. The req could have already been rescheduled on another
worker at this point, causing the req->wrk to be cleared while
processing. Change back to not clearing the pointer for
REQ_FSM_DISEMBARK (it's already been done under mutex in HSH_Lookup
anyways).

Fixes: #1488