CVE-2024-39482: bcache: fix variable length array abuse in btree_iter

In the Linux kernel, the following vulnerability has been resolved:

bcache: fix variable length array abuse in btreeiter

btreeiter is used in two ways: either allocated on the stack with a fixed size MAXBSETS, or from a mempool with a dynamic size based on the specific cache set. Previously, the struct had a fixed-length array of size MAXBSETS which was indexed out-of-bounds for the dynamically-sized iterators, which causes UBSAN to complain.

This patch uses the same approach as in bcachefs's sortiter and splits the iterator into a btreeiter with a flexible array member and a btreeiterstack which embeds a btreeiter as well as a fixed-length data array.