bcachefs: Fix degraded mode fsck

We don't know where the superblock and journal lives on offline devices;
that means if a device is offline fsck can't check those buckets.

Previously, fsck would incorrectly clear bucket data types for those
buckets on offline devices; now we just use the previous state.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -1365,11 +1365,10 @@ static int bch2_alloc_write_key(struct btree_trans *trans,
 {
 	struct bch_fs *c = trans->c;
 	struct bch_dev *ca = bch_dev_bkey_exists(c, iter->pos.inode);
-	struct bucket gc, *b;
+	struct bucket old_gc, gc, *b;
 	struct bkey_i_alloc_v4 *a;
 	struct bch_alloc_v4 old_convert, new;
 	const struct bch_alloc_v4 *old;
-	enum bch_data_type type;
 	int ret;
 
 	old = bch2_alloc_to_v4(k, &old_convert);
@@ -1377,30 +1376,31 @@ static int bch2_alloc_write_key(struct btree_trans *trans,
 
 	percpu_down_read(&c->mark_lock);
 	b = gc_bucket(ca, iter->pos.offset);
+	old_gc = *b;
+
+	if ((old->data_type == BCH_DATA_sb ||
+	     old->data_type == BCH_DATA_journal) &&
+	    !bch2_dev_is_online(ca)) {
+		b->data_type = old->data_type;
+		b->dirty_sectors = old->dirty_sectors;
+	}
 
 	/*
 	 * b->data_type doesn't yet include need_discard & need_gc_gen states -
 	 * fix that here:
 	 */
-	type = __alloc_data_type(b->dirty_sectors,
+	b->data_type = __alloc_data_type(b->dirty_sectors,
 					 b->cached_sectors,
 					 b->stripe,
 					 *old,
 					 b->data_type);
-	if (b->data_type != type) {
-		struct bch_dev_usage *u;
-
-		preempt_disable();
-		u = this_cpu_ptr(ca->usage_gc);
-		u->d[b->data_type].buckets--;
-		b->data_type = type;
-		u->d[b->data_type].buckets++;
-		preempt_enable();
-	}
-
 	gc = *b;
 	percpu_up_read(&c->mark_lock);
 
+	if (gc.data_type != old_gc.data_type ||
+	    gc.dirty_sectors != old_gc.dirty_sectors)
+		bch2_dev_usage_update_m(c, ca, &old_gc, &gc);
+
 	if (metadata_only &&
 	    gc.data_type != BCH_DATA_sb &&
 	    gc.data_type != BCH_DATA_journal &&