The commit dd856dfcb9e74 introduced an optimization that prepends virtio
header to mbuf data. It can be used when the tx mbuf is writeable, so we
need to check that the mbuf is direct (i.e. it embeds its own data).
Fixes: dd856dfcb9e7 ("virtio: use any layout on Tx")
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
88c107840d