sched: window-stats: Additional error checking in sched_set_window()

Check for invalid window size passed as argument to sched_set_window()
Also move up local_irq_disable() call to avoid thread from being
preempted during calculation of window_start and its comparison
against sched_clock(). Use right macro to evluate whether window_start
argument is ahead in time or not.

Change-Id: Idc0d3ab17ede08471ae63b72a2d55e7f84868fd6
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>