net: introduce ptp one step time stamp mode for sync packets

The IEEE 1588 standard (PTP) has a provision for a "one step" mode, where
time stamps on outgoing event packets are inserted into the packet by the
hardware on the fly. This patch adds a new flag for the SIOCSHWTSTAMP
ioctl that lets user space programs request this mode.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>