macvtap: read vnet_hdr_size once

macvtap: read vnet_hdr_size once

[ Upstream commit 837585a5375c38d40361cfe64e6fd11e1addb936 ]

When IFF_VNET_HDR is enabled, a virtio_net header must precede data.
Data length is verified to be greater than or equal to expected header
length tun->vnet_hdr_sz before copying.

Macvtap functions read the value once, but unless READ_ONCE is used,
the compiler may ignore this and read multiple times. Enforce a single
read and locally cached value to avoid updates between test and use.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: BAckported to 3.2:
 - Use ACCESS_ONCE() instead of READ_ONCE()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>