kernel.h: define u8, s8, u32, etc. limits

Create constants that define the maximum and minimum values
representable by the kernel types u8, s8, u16, s16, and so on.

Signed-off-by: Alex Elder <elder@linaro.org>
Cc: Sage Weil <sage@inktank.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>