qla2xxx: Get rid of redundant qla_tgt_sess.tearing_down

The only place that sets qla_tgt_sess.tearing_down calls
target_splice_sess_cmd_list() immediately afterwards, without dropping
the lock it holds.  That function sets se_session.sess_tearing_down,
so we can get rid of the qla_target-specific flag, and in the one
place that looks at the qla_tgt_sess.tearing_down flag just test
se_session.sess_tearing_down instead.

Cc: Chad Dupuis <chad.dupuis@qlogic.com>
Cc: Arun Easi <arun.easi@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>