qla2xxx: fix session free stall by using scst_unregister_session wait=0

Calling scst_unregister_session(wait=1) from qlt_free_session_done
blocks the qla2xxx_wq worker until session teardown completes, but
teardown requires the TM thread to process SCST_UNREG_SESS_TM while
the TM thread is blocked on scst_mutex held by concurrent session
teardown in the global management thread. Under load this stall
exceeds the hung-task timeout. Switch to wait=0 and wait on
fcport->unreg_done instead, matching the pattern in iscsi-scst.

Author: bmeagherix

qla2x00t-32gbit/qla2x00-target/scst_qla2xxx.c

@@ -740,7 +740,23 @@ static void sqa_qla2xxx_free_session(struct fc_port *fcport)
 		DECLARE_COMPLETION_ONSTACK(c);
 
 		fcport->unreg_done = &c;
-		scst_unregister_session(scst_sess, 1, sqa_free_session_done);
+		/*
+		 * Use wait=0 (async) to avoid a severe stall under concurrent
+		 * session teardown.  With wait=1 the workqueue thread blocks
+		 * inside scst_unregister_session() until
+		 * scst_free_session_callback() fires.  That callback cannot be
+		 * scheduled until the TM thread processes SCST_UNREG_SESS_TM,
+		 * but the TM thread is blocked on scst_mutex which the global
+		 * management thread holds during concurrent session teardown
+		 * (scst_sess_free_tgt_devs -> synchronize_rcu()).  Under load
+		 * this stall exceeds the hung-task timeout.  With wait=0 the
+		 * workqueue thread is released immediately; we wait only on
+		 * fcport->unreg_done, signalled by sqa_free_session_done() at
+		 * a point where scst_mutex is not held.
+		 * See also: iscsi-scst session_free() which carries a similar
+		 * warning.
+		 */
+		scst_unregister_session(scst_sess, 0, sqa_free_session_done);
 		wait_for_completion(&c);
 	}