KVM: x86: Fix update RCX/RDI/RSI on REP-string

When REP-string instruction is preceded with an address-size prefix,
ECX/EDI/ESI are used as the operation counter and pointers.  When they are
updated, the high 32-bits of RCX/RDI/RSI are cleared, similarly to the way they
are updated on every 32-bit register operation.  Fix it.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -524,13 +524,9 @@ static void masked_increment(ulong *reg, ulong mask, int inc)
 static inline void
 register_address_increment(struct x86_emulate_ctxt *ctxt, int reg, int inc)
 {
-	ulong mask;
+	ulong *preg = reg_rmw(ctxt, reg);
 
-	if (ctxt->ad_bytes == sizeof(unsigned long))
-		mask = ~0UL;
-	else
-		mask = ad_mask(ctxt);
-	masked_increment(reg_rmw(ctxt, reg), mask, inc);
+	assign_register(preg, *preg + inc, ctxt->ad_bytes);
 }
 
 static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)