time: Clean up warp_clock()

warp_clock() currently accesses timekeeping internal state directly, which
is unnecessary.  Convert it to use the proper timekeeping interfaces.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>