KVM: fix an if() condition

It might have worked in this case since PT_PRESENT_MASK is 1, but let's
express this correctly.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index 32c64f6..e8e2281 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -1408,7 +1408,7 @@
 	for (i = 0; i < PT64_ENT_PER_PAGE; ++i, va += va_delta) {
 		u64 ent = pt[i];
 
-		if (!ent & PT_PRESENT_MASK)
+		if (!(ent & PT_PRESENT_MASK))
 			continue;
 
 		va = canonicalize(va);