bcachefs: Track the maximum btree_paths ever allocated by each transaction

bcachefs: Track the maximum btree_paths ever allocated by each transaction

We need a way to check if the machinery for handling btree_paths with in
a transaction is behaving reasonably, as it often has not been - we've
had bugs with transaction path overflows caused by duplicate paths and
plenty of other things.

This patch tracks, per transaction fn, the most btree paths ever
allocated by that transaction and makes it available in debugfs.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>