KVM: x86/mmu: Avoid modulo operator on 64-bit value to fix i386 build

Replace a modulo operator with the more common pattern for computing the
gfn "offset" of a huge page to fix an i386 build error.

  arch/x86/kvm/mmu/tdp_mmu.c:212: undefined reference to `__umoddi3'

In fact, almost all of tdp_mmu.c can be elided on 32-bit builds, but
that is a much larger patch.

Fixes: 2f2fad08 ("kvm: x86/mmu: Add functions to handle changed TDP SPTEs")
Reported-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20201024031150.9318-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -209,7 +209,7 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 
 	WARN_ON(level > PT64_ROOT_MAX_LEVEL);
 	WARN_ON(level < PG_LEVEL_4K);
-	WARN_ON(gfn % KVM_PAGES_PER_HPAGE(level));
+	WARN_ON(gfn & (KVM_PAGES_PER_HPAGE(level) - 1));
 
 	/*
 	 * If this warning were to trigger it would indicate that there was a