bcachefs: Improve bch2_check_alloc_info

bcachefs: Improve bch2_check_alloc_info

 - In check_alloc_key(), previously we were re-initializing iterators
   for the need_discard and freespace btrees for every alloc key we
   checked. But this was causing us to redo lookups into the journal
   keys every time, since those lookups are cached in struct btree_iter.
   This initializes the iterators in bch2_check_alloc_info and passes
   them into check_alloc_key().

 - Make the looping more consistent/efficient in bch2_check_alloc_info()

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