librbd: add rbd_snap_get_trash_namespace2() API to return full namespace

librbd: add rbd_snap_get_trash_namespace2() API to return full namespace

The existing rbd_snap_get_trash_namespace() API returns only the
original name of the deleted snapshot, omitting its namespace type.
While non-user snapshots have distinctive names, there is nothing
preventing the user from creating user snapshots with identical names
(i.e. starting with ".group" or ".mirror" prefix).  After cloning from
non-user snapshots is allowed, it's possible for such user snapshots to
get mixed up with non-user snapshots in the trash, so let's provide
means for disambiguation.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>