bcachefs: journal keys: sort keys for interior nodes first

bcachefs: journal keys: sort keys for interior nodes first

There's an unavoidable issue with btree lookups when we're overlaying
journal keys and the journal has many deletions for keys present in the
btree - peek operations will have to iterate over all those deletions to
find the next live key to return.

This is mainly a problem for lookups in interior nodes, if we have to
traverse to a leaf. Looking up an insert position in a leaf (for journal
replay) doesn't have to find the next live key, but walking down the
btree does.

So to ameloriate this, change journal key sort ordering so that we
replay keys from roots and interior nodes first.

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