tick-sched: recalculate sleep length each time it is requested

Recalculating the sleep length each time its called allows us to account
for the fact that the amount of time we can sleep for might change after
tick_nohz_stop_sched_tick is called in idle. The prime example of this
is an idle notifier that cancels timers as we are entering idle.

Change-Id: I92871efc7befb3fee2a816da16145ba9da334a9e
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>