sched: Prevent wakeup to enter critical section needlessly

Check the waitqueue task list to be non empty before entering the critical
section. This prevents locking the spin lock needlessly in case the queue
was empty, and therefor also prevent scheduling overhead on a PREEMPT_RT
system.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
backported from Linux 3.5+ by faux123
timers: Avoid finding cpu for migration of running timers in __mod_timer()

Currently we don't support migration of currently running timers. Current code
flow in __mod_timer() is:
- Find a cpu where we should migrate the timer
- Check if timer is currently running
- If yes, don't migrate.

In this process, the first step is a unnecessary activiy, if the timer is
currently running. This patch tries to avoid it in such cases.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
backported from Linux 3.5+ by faux123