selftests/ptp: Fix timestamp printf format for PTP_SYS_OFFSET

Previously, timestamps were printed using "%lld.%u" which is incorrect
for nanosecond values lower than 100,000,000 as they're fractional
digits, therefore leading zeros are meaningful.

This patch changes the format strings to "%lld.%09u" in order to add
leading zeros to the nanosecond value.

Fixes: 568ebc59 ("ptp: add the PTP_SYS_OFFSET ioctl to the testptp program")
Fixes: 4ec54f95 ("ptp: Fix compiler warnings in the testptp utility")
Fixes: 6ab0e475

 ("Documentation: fix misc. warnings")
Signed-off-by: Alex Maftei <alex.maftei@amd.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://lore.kernel.org/r/20230615083404.57112-1-alex.maftei@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>