There are two issues with the checks against VM_MAXUSER_ADDRESS. First,
the comparison should consider the values as unsigned, otherwise
addresses with the high bit set will fail to branch. Second, the value
of VM_MAXUSER_ADDRESS is, by convention, one larger than the maximum
mappable user address and invalid itself. Thus, use the bgeu instruction
for these comparisons.
Reported by: Robert Morris <firstname.lastname@example.org>