libceph: validate blob_struct_v in process_one_ticket()

None of these are validated in userspace, but since we do validate
reply_struct_v in ceph_x_proc_ticket_reply(), tkt_struct_v (first) and
CephXServiceTicket struct_v (second) in process_one_ticket(), validate
CephXTicketBlob struct_v as well.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>