bch2_btree_and_journal_walk() walks the btree overlaying keys from the
journal; it was introduced so that we could read in the alloc btree
prior to journal replay being done, when journalling of updates to
interior btree nodes was introduced.

But it didn't have btree node prefetching, which introduced a severe
regression with mount times, particularly on spinning rust. This patch
implements btree node prefetching for the btree + journal walk,
hopefully fixing that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>