bcachefs: Fix journal_keys_search() overhead

Previously, on every btree_iter_peek() operation we were searching the
journal keys, doing a full binary search - which was slow.

This patch fixes that by saving our position in the journal keys, so
that we only do a full binary search when moving our position backwards
or a large jump forwards.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>