ptp: add the PTP_SYS_OFFSET ioctl to the testptp program

This patch add a method into testptp.c to measure the time offset
between phc and system clock through the ioctl PTP_SYS_OFFSET.

Signed-off-by: Dong Zhu <bluezhudong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>