MDEV-22827 InnoDB: Failing assertion: purge_sys->n_stop == 0

When MDEV-22769 introduced srv_shutdown_state=SRV_SHUTDOWN_INITIATED in
commit efc70da
we forgot to adjust a few checks for SRV_SHUTDOWN_NONE.

In the initial shutdown step, we are waiting for the background
DROP TABLE queue to be processed or discarded. At that time,
some background tasks (such as buffer pool resizing or dumping
or encryption key rotation) may be terminated, but others must
remain running normally.

srv_purge_coordinator_suspend(), srv_purge_coordinator_thread(),
srv_start_wait_for_purge_to_start(): Treat SRV_SHUTDOWN_NONE
and SRV_SHUTDOWN_INITIATED equally.

storage/innobase/srv/srv0srv.cc

@@ -2780,7 +2780,7 @@ srv_purge_coordinator_suspend(
 
 		rw_lock_x_lock(&purge_sys->latch);
 
-		stop = (srv_shutdown_state == SRV_SHUTDOWN_NONE
+		stop = (srv_shutdown_state <= SRV_SHUTDOWN_INITIATED
 			&& purge_sys->state == PURGE_STATE_STOP);
 
 		if (!stop) {
@@ -2856,7 +2856,7 @@ DECLARE_THREAD(srv_purge_coordinator_thread)(
 		/* If there are no records to purge or the last
 		purge didn't purge any records then wait for activity. */
 
-		if (srv_shutdown_state == SRV_SHUTDOWN_NONE
+		if (srv_shutdown_state <= SRV_SHUTDOWN_INITIATED
 		    && srv_undo_sources
 		    && (purge_sys->state == PURGE_STATE_STOP
 			|| n_total_purged == 0)) {

storage/innobase/srv/srv0start.cc

@@ -1092,7 +1092,7 @@ srv_start_wait_for_purge_to_start()
 
 	ut_a(state != PURGE_STATE_DISABLED);
 
-	while (srv_shutdown_state == SRV_SHUTDOWN_NONE
+	while (srv_shutdown_state <= SRV_SHUTDOWN_INITIATED
 	       && srv_force_recovery < SRV_FORCE_NO_BACKGROUND
 	       && state == PURGE_STATE_INIT) {