bcache: Fix for handling overlapping extents when reading in a btree node

/* Mergesort */

static void sort_key_next(struct btree_iter *iter,

			  struct btree_iter_set *i)

{

	i->k = bkey_next(i->k);

	if (i->k == i->end)

		*i = iter->data[--iter->used];

}

static void btree_sort_fixup(struct btree_iter *iter)

{

	while (iter->used > 1) {

		struct btree_iter_set *top = iter->data, *i = top + 1;

		struct bkey *k;

		if (iter->used > 2 &&

		    btree_iter_cmp(i[0], i[1]))

			i++;

		for (k = i->k;

		     k != i->end && bkey_cmp(top->k, &START_KEY(k)) > 0;

		     k = bkey_next(k))

			if (top->k > i->k)

				__bch_cut_front(top->k, k);

			else if (KEY_SIZE(k))

				bch_cut_back(&START_KEY(k), top->k);

		if (top->k < i->k || k == i->k)

		if (bkey_cmp(top->k, &START_KEY(i->k)) <= 0)

			break;

		if (!KEY_SIZE(i->k)) {

			sort_key_next(iter, i);

			heap_sift(iter, i - top, btree_iter_cmp);

			continue;

		}

		if (top->k > i->k) {

			if (bkey_cmp(top->k, i->k) >= 0)

				sort_key_next(iter, i);

			else

				bch_cut_front(top->k, i->k);

			heap_sift(iter, i - top, btree_iter_cmp);

		} else {

			/* can't happen because of comparison func */

			BUG_ON(!bkey_cmp(&START_KEY(top->k), &START_KEY(i->k)));

			bch_cut_back(&START_KEY(i->k), top->k);

		}

	}

}