lib/checksum.c: optimize do_csum a bit
Reduce the number of variables modified by the loop in do_csum() by 1,
which seems like a good idea.  On Nios II (a RISC CPU with 3-operand
instruction set) it reduces the loop from 7 to 6 instructions, including
the conditional branch.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

Orig patch: https://github.com/torvalds/linux/commit/be0e1e788b0973147b6b83cc940676cc26009eb7