[MIPS] Fix 64-bit IP checksum code

Use unsigned loads to avoid possible misscalculation of IP checksums.  This
bug was instruced in f761106cd728bcf65b7fe161b10221ee00cf7132 (lmo) /
ed99e2bc

 (kernel.org).

[Original fix by Atsushi.  Improved instruction scheduling and fix for
unaligned unsigned load by me -- Ralf]

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>