lib/checksum.c: use 32-bit arithmetic consistently

The use of 'unsigned long' variables in the 32-bit part of do_csum()
is confusing at best, and potentially broken for long input on 64-bit
machines.

This changes the code to use 'unsigned int' instead, which makes
the code behave in the same (correct) way on both 32 and 64 bit
machines.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>