ceph: fix incorrectly showing the .snap size for stat

We should set the 'stat->size' to the real number of snapshots for
snapdirs.

Link: https://tracker.ceph.com/issues/57342


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>