net/virtio: fix Rx scatter offload

net/virtio: fix Rx scatter offload

Report Rx scatter offload capability depending on VIRTIO_NET_F_MRG_RXBUF.

If Rx scatter is not requested, ensure that provided Rx buffers on
each Rx queue are big enough to fit Rx packets up to configured MTU.

Fixes: ce17eddefc20 ("ethdev: introduce Rx queue offloads API")
Cc: stable@dpdk.org
Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>