proc/stat: use defined macro KMALLOC_MAX_SIZE

There is a macro for the max size kmalloc can allocate, so use it instead
of a hardcoded number.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>