bcachefs: Fix transaction restarts due to upgrading of cloned iterators

bcachefs: Fix transaction restarts due to upgrading of cloned iterators

This fixes a regression from
  52d86202fd bcachefs: Improve bch2_btree_iter_traverse_all()

We want to avoid mucking with other iterators in the btree transaction
in operations that are only supposed to be touching individual iterators
- that patch was a cleanup to move lock ordering handling to
bch2_btree_iter_traverse_all(). But it broke upgrading of cloned
iterators.

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