net/virtio: fix indirect desc length

net/virtio: fix indirect desc length

When transmitting indirect descriptors, first desc will store net_hdr
and following descs will be mapped to mbuf segments. Total desc number
will be seg_num plus one. Meaning of variable needed is the number of
used descs in packed ring. This value will always be two for indirect
desc. Now use mbuf segments number for calculating correct desc length.

Fixes: b473061b0e1d ("net/virtio: fix indirect descriptors in packed datapaths")
Cc: stable@dpdk.org
Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>