Compute the range length as end minus start. The reversed unsigned
subtraction underflowed for nonempty ranges, selecting a full-map
TLB invalidation even when at most eight pages needed invalidation.
Fixes: 65bbba25d214 ("powerpc64: Implement Radix MMU for POWER9 CPUs")
MFC after: 2 weeks
Sponsored by: FreeBSD Foundation, Reliable Computer Systems Lab