sheep: read timestmap with correct size in do_epoch_log_read()

do_epoch_log_read() reads timestamp with sizeof(timestamp). But the
size should be sizeof(*timestamp). This would cause problems on 32bit
architectures.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>