dpdk: patch broken virtio macro

dpdk: patch broken virtio macro

Type: fix

In DPDK 20.08, the macro ASSIGN_UNLESS_EQUAL(), which is defined in the
virtio PMD code, is broken. It does not actually set the variable that is
passed in to the value that is passed in. It just assigns the value
to a local variable that it declares. This results in
virtqueue_clear_net_hdr() failing to clear the fields in the virtio
net header. This can cause packets which were sent over an IPsec tunnel
and are subsequently transmitted on a virtio interface to have garbage
in the virtio net header.

Apply a post-20.08 patch from upstream to fix it.

Change-Id: Ic53e89d859db628e26a3c1988e53b626fede94d7
Signed-off-by: Matthew Smith <mgsmith@netgate.com>