bcachefs: Fix key cache + BTREE_ITER_all_snapshots

bcachefs: Fix key cache + BTREE_ITER_all_snapshots

Normally, whitouts (KEY_TYPE_whitout) are filtered from btree lookups,
since they exist only to represent deletions of keys in ancestor
snapshots - except, they should not be filtered in
BTREE_ITER_all_snapshots mode, so that e.g. snapshot deletion can clean
them up.

This means that that the key cache has to store whiteouts, and key cache
fills cannot filter them.

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