bcachefs: Update export_operations for snapshots

When support for snapshots was merged, export operations weren't
updated yet. This patch adds new filehandle types for bcachefs that
include the subvolume ID and updates export operations for subvolumes -
and also .get_parent, support for which was added just prior to
snapshots.

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