bcachefs: Split btree_iter_traverse and bch2_btree_iter_traverse()

bcachefs: Split btree_iter_traverse and bch2_btree_iter_traverse()

External (to the btree iterator code) users of bch2_btree_iter_traverse
expect that on success the iterator will be pointed at iter->pos and
have that position locked - but since we split iter->pos and
iter->real_pos, that means it has to update iter->real_pos if necessary.

Internal users don't expect it to modify iter->real_pos, so we need two
separate functions.

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