proc: read kernel cpu stat pointer once

Help gcc generate better code:

	$ ./scripts/bloat-o-meter ../vmlinux-000 ../vmlinux-001
	add/remove: 2/2 grow/shrink: 0/1 up/down: 92/-142 (-50)
	Function                                     old     new   delta
	get_iowait_time.isra                           -      46     +46
	get_idle_time.isra                             -      46     +46
	show_stat                                   1489    1477     -12
	get_iowait_time                               65       -     -65
	get_idle_time                                 65       -     -65

Link: http://lkml.kernel.org/r/20190114195907.GA9680@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>