bcachefs: Fix debug assert

Reported-by: syzbot+a8074a75b8d73328751e@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -564,7 +564,7 @@ static void __bch2_fs_free(struct bch_fs *c)
 	BUG_ON(atomic_read(&c->journal_keys.ref));
 	bch2_fs_btree_write_buffer_exit(c);
 	percpu_free_rwsem(&c->mark_lock);
-	EBUG_ON(percpu_u64_get(c->online_reserved));
+	EBUG_ON(c->online_reserved && percpu_u64_get(c->online_reserved));
 	free_percpu(c->online_reserved);
 
 	darray_exit(&c->btree_roots_extra);