tap: reset vnet header size on open

For tap, we currently assume the vnet header size is 10
(the default value) but that might not be the case
if tap is persistent and has been used by qemu previously.
To fix, set host header size in tap device on open.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Tested-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

net/tap.c

@@ -341,6 +341,13 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
     s->using_vnet_hdr = 0;
     s->has_ufo = tap_probe_has_ufo(s->fd);
     tap_set_offload(&s->nc, 0, 0, 0, 0, 0);
+    /*
+     * Make sure host header length is set correctly in tap:
+     * it might have been modified by another instance of qemu.
+     */
+    if (tap_probe_vnet_hdr_len(s->fd, s->host_vnet_hdr_len)) {
+        tap_fd_set_vnet_hdr_len(s->fd, s->host_vnet_hdr_len);
+    }
     tap_read_poll(s, 1);
     s->vhost_net = NULL;
     return s;