sysctl/sched: do not allow value "0" for sched_{up,down}_migrate knobs

Right now sched_{up,down}_migrate knobs are accepting zero as a valid
value and we end up with upmigrate value is less than downmigrate value
when zero is written by user to upmigrate knob, we go ahead and update
upmigrate value which would end up being lesser than downmigrate value.
Also, there is a div by zero issue by accepting zero as valid value for
these knobs.

So, update check to make sure zero isn't allowed as a valid value.

Change-Id: Ic44d8fda14e99f622bbadc6c78ce7a28aedbd5b2
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>