net: tap: fix wrong transport_header when sending VLAN-tagged frame

net: tap: fix wrong transport_header when sending VLAN-tagged frame

In tap_get_user_xdp(), when processing a VLAN-tagged frame (e.g.
ETH_P_8021Q), skb_set_network_header() is called first to advance
network_header past the VLAN tag to the inner protocol header.
skb_probe_transport_header() is then called with skb->protocol still
set to ETH_P_8021Q, while nhoff (derived from skb_network_offset())
already points past the VLAN tag to the inner protocol header.

In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff
points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it
reads a struct vlan_hdr at the current nhoff via __skb_header_pointer(),
but that offset contains the inner protocol header (e.g. an IP header).
The bytes are misinterpreted as a VLAN header, yielding a garbage
encapsulated EtherType that matches no known protocol. The dissector
returns false, so skb_probe_transport_header() never calls
skb_set_transport_header(), leaving transport_header at its uninitialized
sentinel value (~0U).

Move skb_set_network_header() to after skb_probe_transport_header(). At
the time skb_probe_transport_header() is called, network_header still
points to the VLAN header (offset ETH_HLEN), so nhoff is correct and the
flow dissector can parse the VLAN header, extract the inner EtherType,
and advance nhoff to the inner protocol header, allowing transport_header
to be set correctly.

Fixes: 8c76e77f9069 ("tap: call skb_probe_transport_header after setting skb->dev")
Assisted-by: WChat:claude-opus-4-8
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260807063405.688780-3-wei.fang@oss.nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>