net/virtio: fix compilation with -Og

net/virtio: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC virtio_rxtx.o
  virtio_rxtx.c: In function ‘virtio_rx_offload’:
  virtio_rxtx.c:680:10: error: ‘csum’ may be used uninitialized in
                        this function [-Werror=maybe-uninitialized]
       csum = ~csum;
       ~~~~~^~~~~~~

The function rte_raw_cksum_mbuf() may indeed return an error, and
in this case, csum won't be initialized. Fix it by initializing csum
to 0.

Fixes: 96cb6711939e ("net/virtio: support Rx checksum offload")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>