bcachefs: Replace bch2_btree_iter_next() calls with bch2_btree_iter_advance

bcachefs: Replace bch2_btree_iter_next() calls with bch2_btree_iter_advance

The way btree iterators work internally has been changing, particularly
with the iter->real_pos changes, and bch2_btree_iter_next() is no longer
hyper optimized - it's just advance followed by peek, so it's more
efficient to just call advance where we're not using the return value of
bch2_btree_iter_next().

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