lib: dhry: use ktime_ms_delta() helper

Use the existing ktime_ms_delta() helper instead of open-coding the same
operation.

Link: https://lkml.kernel.org/r/bb43c67a7580de6152f5e6eb225071166d33b6e4.1705934853.git.geert+renesas@glider.be


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>