vhost: use struct_size and size_add to compute flex array sizes

The vhost_get_avail_size and vhost_get_used_size functions compute the size
of structures with flexible array members with an additional 2 bytes if the
VIRTIO_RING_F_EVENT_IDX feature flag is set. Convert these functions to use
struct_size() and size_add() instead of coding the calculation by hand.

This ensures that the calculations will saturate at SIZE_MAX rather than
overflowing.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: kvm@vger.kernel.org
Message-Id: <20230227214127.3678392-1-jacob.e.keller@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>