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

tools/testing/selftests/ptp/testptp.c

@@ -502,11 +502,11 @@ int main(int argc, char *argv[])
 			interval = t2 - t1;
 			offset = (t2 + t1) / 2 - tp;
 
-			printf("system time: %lld.%u\n",
+			printf("system time: %lld.%09u\n",
 				(pct+2*i)->sec, (pct+2*i)->nsec);
-			printf("phc    time: %lld.%u\n",
+			printf("phc    time: %lld.%09u\n",
 				(pct+2*i+1)->sec, (pct+2*i+1)->nsec);
-			printf("system time: %lld.%u\n",
+			printf("system time: %lld.%09u\n",
 				(pct+2*i+2)->sec, (pct+2*i+2)->nsec);
 			printf("system/phc clock time offset is %" PRId64 " ns\n"
 			       "system     clock time delay  is %" PRId64 " ns\n",