bcachefs: Fix bch2_trans_relock()

bcachefs: Fix bch2_trans_relock()

The patch that changed bch2_trans_relock() to not look at iter->uptodate
also tried to add an optimization by only having it relock
btree_iter_key() iterators (iterators that are live or have been marked
as keep). But, this wasn't thought through - this pops internal iterator
assertions because on transaction restart, when we're traversing
iterators we traverse all iterators marked as linked, and having
bch2_trans_relock() skip some of those mean that it can skil the
iterator that bch2_btree_iter_traverse_one() is currently traversing.

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