gfs2: fix trans slab error when withdraw occurs inside log_flush

Log flush operations (gfs2_log_flush()) can target a specific transaction.
But if the function encounters errors (e.g. io errors) and withdraws,
the transaction was only freed it if was queued to one of the ail lists.
If the withdraw occurred before the transaction was queued to the ail1
list, function ail_drain never freed it. The result was:

BUG gfs2_trans: Objects remaining in gfs2_trans on __kmem_cache_shutdown()

This patch makes log_flush() add the targeted transaction to the ail1
list so that function ail_drain() will find and free it properly.

Cc: stable@vger.kernel.org # v5.7+
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/log.c

@@ -1002,6 +1002,16 @@ void gfs2_log_flush(struct gfs2_sbd *sdp, struct gfs2_glock *gl, u32 flags)
 
 out:
 	if (gfs2_withdrawn(sdp)) {
+		/**
+		 * If the tr_list is empty, we're withdrawing during a log
+		 * flush that targets a transaction, but the transaction was
+		 * never queued onto any of the ail lists. Here we add it to
+		 * ail1 just so that ail_drain() will find and free it.
+		 */
+		spin_lock(&sdp->sd_ail_lock);
+		if (tr && list_empty(&tr->tr_list))
+			list_add(&tr->tr_list, &sdp->sd_ail1_list);
+		spin_unlock(&sdp->sd_ail_lock);
 		ail_drain(sdp); /* frees all transactions */
 		tr = NULL;
 	}