testptp: add option to shift clock by nanoseconds

Add option to shift the clock by a specified number of nanoseconds.

The new argument -n will specify the number of nanoseconds to add to the
ptp clock. Since the API doesn't support negative shifts those needs to
be calculated by subtracting full seconds and adding a nanosecond offset.

Signed-off-by: Maciek Machnikowski <maciek@machnikowski.net>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://lore.kernel.org/r/20220221200637.125595-1-maciek@machnikowski.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>