ntp: use unsigned input for do_div()

The kernel NTP code shouldn't hand 64-bit *signed* values to do_div().  Make it
instead hand 64-bit unsigned values.  This gets rid of a couple of warnings.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>