sched/nohz: Fix overflow error in scheduler_tick_max_deferment()

sched/nohz: Fix overflow error in scheduler_tick_max_deferment()

While calculating the scheduler tick max deferment, the delta is
converted from microseconds to nanoseconds through a multiplication
against NSEC_PER_USEC.

But this microseconds operand is an unsigned int, thus the result may
likely overflow. The result is cast to u64 but only once the operation
is completed, which is too late to avoid overflown result.

This is currently not a problem because the scheduler tick max deferment
is 1 second. But this may become an issue as we plan to make this
value tunable.

So lets fix this by casting the usecs value to u64 before multiplying by
NSECS_PER_USEC.

Also to prevent from this kind of mistake to happen again, move this
ad-hoc jiffies -> nsecs conversion to a new helper.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alex Shi <alex.shi@linaro.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Kevin Hilman <khilman@linaro.org>
Link: http://lkml.kernel.org/r/1387315388-31676-2-git-send-email-khilman@linaro.org
[move ad-hoc conversion to jiffies_to_nsecs helper]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>