cfiscsi_done: Free the dummy PDU earlier.

The dummy PDU needs to be freed before marking task abortion complete
as otherwise cfiscsi_session_terminate_tasks can return and destroy
the session in another thread before the PDU is freed.

Fixes: 2e8d1a55258d iscsi: Allocate a dummy PDU for the internal nexus reset task.
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34176

(cherry picked from commit a3d71fffa78619fb394b8bb595d2ef680bd0e43a)