ntp: Fix RTC synchronization on 32-bit platforms

Due to an integer overflow, RTC synchronization now happens every 2s
instead of the intended 11 minutes.  Fix this by forcing 64-bit
arithmetic for the sync period calculation.

Annotate the other place which multiplies seconds for consistency as well.

Fixes: c9e6189f

 ("ntp: Make the RTC synchronization more reliable")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210111103956.290378-1-geert+renesas@glider.be