bcachefs: Implement bch2_btree_iter_prev_min()

bcachefs: Implement bch2_btree_iter_prev_min()

A user contributed a filessytem dump, where the dump was actually
corrupted (due to being taken while the filesystem was online), but
which exposed an interesting bug in fsck - reconstruct_inode().

When itearting in BTREE_ITER_filter_snapshots mode, it's required to
give an end position for the iteration and it can't span inode numbers;
continuing into the next inode might mean we start seeing keys from a
different snapshot tree, that the is_ancestor() checks always filter,
thus we're never able to return a key and stop iterating.

Backwards iteration never implemented the end position because nothing
else needed it - except for reconstuct_inode().

Additionally, backwards iteration is now able to overlay keys from the
journal, which will be useful if we ever decide to start doing journal
replay in the background.

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