sched/sysctl: check for min/max values for sched_{up,down}_migrate knobs

Now that the sched_{up,down}_migrate knobs accept multiple values
in case of multi cluster systems, make sure each value supplied
by user is with in the range. Otherwise, user would see weird
behavior when the knobs are read back after writing values
(especially zero for downmigrate) supplied because one of
the value supplied would be updated.

For example:
	# cat /proc/sys/kernel/sched_downmigrate
	85 85
	# cat /proc/sys/kernel/sched_upmigrate
	95 95
	# echo "0 0" > /proc/sys/kernel/sched_downmigrate
	# echo "100 100" > /proc/sys/kernel/sched_upmigrate
	# cat /proc/sys/kernel/sched_downmigrate
	85 0
	# cat /proc/sys/kernel/sched_upmigrate
	95 100

Expected Results:
	# cat /proc/sys/kernel/sched_downmigrate
	0 0
	# cat /proc/sys/kernel/sched_upmigrate
	100 100

Change-Id: Id6db51656b71c3399ecd16ff5ef7c85ced6d7ceb
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>