KVM: X86: use vzalloc() instead of vmalloc/memset

fixed the following warning：
/virt/kvm/dirty_ring.c:70:20-27: WARNING: vzalloc should be used for
ring -> dirty_gfns, instead of vmalloc/memset.
Signed-off-by: NTian Tao <tiantao6@hisilicon.com>
Message-Id: <1611547045-13669-1-git-send-email-tiantao6@hisilicon.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

virt/kvm/dirty_ring.c

@@ -67,10 +67,9 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
 
 int kvm_dirty_ring_alloc(struct kvm_dirty_ring *ring, int index, u32 size)
 {
-	ring->dirty_gfns = vmalloc(size);
+	ring->dirty_gfns = vzalloc(size);
 	if (!ring->dirty_gfns)
 		return -ENOMEM;
-	memset(ring->dirty_gfns, 0, size);
 
 	ring->size = size / sizeof(struct kvm_dirty_gfn);
 	ring->soft_limit = ring->size - kvm_dirty_ring_get_rsvd_entries();