KVM: MMU: Rename PT64_ROOT_LEVEL to PT64_ROOT_4LEVEL.

Now we have 4 level page table and 5 level page table in 64 bits
long mode, let's rename the PT64_ROOT_LEVEL to PT64_ROOT_4LEVEL,
then we can use PT64_ROOT_5LEVEL for 5 level page table, it's
helpful to make the code more clear.

Also PT64_ROOT_MAX_LEVEL is defined as 4, so that we can just
redefine it to 5 whenever a replacement is needed for 5 level
paging.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index c5b9705..7fc64a5 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -584,7 +584,7 @@ static inline void invlpga(unsigned long addr, u32 asid)
 static int get_npt_level(void)
 {
 #ifdef CONFIG_X86_64
-	return PT64_ROOT_LEVEL;
+	return PT64_ROOT_4LEVEL;
 #else
 	return PT32E_ROOT_LEVEL;
 #endif