bcachefs: Fix reference to invalid bucket in copygc

Use bch2_dev_bucket_tryget() instead of bch2_dev_tryget() before
checking the bucket bitmap.

Reported-by: syzbot+3168625f36f4a539237e@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/movinggc.c

@@ -71,7 +71,7 @@ static int bch2_bucket_is_movable(struct btree_trans *trans,
 	if (ret)
 		return ret;
 
-	struct bch_dev *ca = bch2_dev_tryget(c, k.k->p.inode);
+	struct bch_dev *ca = bch2_dev_bucket_tryget(c, k.k->p);
 	if (!ca)
 		goto out;