librbd: add rbd_clone4() API to take parent snapshot by ID

librbd: add rbd_clone4() API to take parent snapshot by ID

Allow cloning from non-user snapshots -- namely snapshots in group
and mirror namespaces.  The motivation is to provide a building block
for cloning new groups from group snapshots ("rbd group snap create").
Otherwise, group snapshots as they are today can be used only for
rolling back the group as a whole, which is very limiting.

While at it, there doesn't seem to be anything wrong with making it
possible to clone from mirror snapshots as well.

Snapshots in a trash namespace can't be cloned from since they are
considered to be deleted.

Cloning from non-user snapshots is limited to clone v2 just because
protecting/unprotecting is limited to snapshots in a user namespace.
This happens to simplify some invariants.

Fixes: https://tracker.ceph.com/issues/64662
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>