sched/x86: Fix overflow in cyc2ns_offset

sched/x86: Fix overflow in cyc2ns_offset

When a machine boots up, the TSC generally gets reset.  However,
when kexec is used to boot into a kernel, the TSC value would be
carried over from the previous kernel.  The computation of
cycns_offset in set_cyc2ns_scale is prone to an overflow, if the
machine has been up more than 208 days prior to the kexec.  The
overflow happens when we multiply *scale, even though there is
enough room to store the final answer.

We fix this issue by decomposing tsc_now into the quotient and
remainder of division by CYC2NS_SCALE_FACTOR and then performing
the multiplication separately on the two components.

Refactor code to share the calculation with the previous
fix in __cycles_2_ns().

Signed-off-by: Salman Qazi <[email protected]>
Acked-by: John Stultz <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: john stultz <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>