virtio_net: Cleanup '2+MAX_SKB_FRAGS'

Improve consistency by using everywhere it is needed
'MAX_SKB_FRAGS + 2' rather than '2+MAX_SKB_FRAGS' or
'2 + MAX_SKB_FRAGS'.

No functional change.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250521092236.661410-3-lvivier@redhat.com


Tested-by: Lei Yang <leiyang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>