make afsd partition size check be large partition safe

make afsd partition size check be large partition safe

right now the math overflows the 32 bit type for >2tb partitions.
use larger variables to avoid this.

Reviewed-on: http://gerrit.openafs.org/392
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>