bcachefs: Don't issue discards when in nochanges mode

When the nochanges option is selected, we're supposed to never issue
writes. Unfortunately, it seems discards were missed when implemnting
this, leading to some painful filesystem corruption.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/alloc_background.c

@@ -801,7 +801,8 @@ static int push_invalidated_bucket(struct bch_fs *c, struct bch_dev *ca, u64 b)
 
 static void discard_one_bucket(struct bch_fs *c, struct bch_dev *ca, u64 b)
 {
-	if (ca->mi.discard &&
+	if (!c->opts.nochanges &&
+	    ca->mi.discard &&
 	    bdev_max_discard_sectors(ca->disk_sb.bdev))
 		blkdev_issue_discard(ca->disk_sb.bdev, bucket_to_sector(ca, b),
 				     ca->mi.bucket_size, GFP_NOFS);

fs/bcachefs/journal_reclaim.c

@@ -286,7 +286,8 @@ void bch2_journal_do_discards(struct journal *j)
 		struct journal_device *ja = &ca->journal;
 
 		while (should_discard_bucket(j, ja)) {
-			if (ca->mi.discard &&
+			if (!c->opts.nochanges &&
+			    ca->mi.discard &&
 			    bdev_max_discard_sectors(ca->disk_sb.bdev))
 				blkdev_issue_discard(ca->disk_sb.bdev,
 					bucket_to_sector(ca,