The new backpointers code will be using bch2_btree_iter_peek_slot() on
interior nodes - this patch updates peek_slot() to make that work.

 - Pass the correct level to bch2_journal_keys_peek_slot()
 - We should only set BTREE_ITER_CACHED or BTREE_ITER_WITH_KEY_CACHE
   when using bch2_trans_iter_init(), not bch2_trans_node_iter_init()
 - Update assertions
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>