0xffffffff / (0xffffffff / n) can have a value as large as n (consider the
n == 1 case), but we need a value no bigger than n-1. So add 1 before
dividing to fix the problem.
This caused a test failure on Debian "lenny" amd64 when apparently
unrelated code changed.
Reported-by: Justin Pettit <jpettit@nicira.com>