bcachefs: BTREE_ITER_FILTER_SNAPSHOTS

For snapshots, we need to implement btree lookups that return the first
key that's an ancestor of the snapshot ID the lookup is being done in -
and filter out keys in unrelated snapshots. This patch adds the btree
iterator flag BTREE_ITER_FILTER_SNAPSHOTS which does that filtering.

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