net: 802: LLC+SNAP OID:PID lookup on start of skb data

net: 802: LLC+SNAP OID:PID lookup on start of skb data

802.2+LLC+SNAP frames received by napi_complete_done() with GRO and DSA
have skb->transport_header set two bytes short, or pointing 2 bytes
before network_header & skb->data. This was an issue as snap_rcv()
expected offset to point to SNAP header (OID:PID), causing packet to
be dropped.

A fix at llc_fixup_skb() (a024e377efed) resets transport_header for any
LLC consumers that may care about it, and stops SNAP packets from being
dropped, but doesn't fix the problem which is that LLC and SNAP should
not use transport_header offset.

Ths patch eliminates the use of transport_header offset for SNAP lookup
of OID:PID so that SNAP does not rely on the offset at all.
The offset is reset after pull for any SNAP packet consumers that may
(but shouldn't) use it.

Fixes: fda55eca5a33 ("net: introduce skb_transport_header_was_set()")
Signed-off-by: Antonio Pastor <antonio.pastor@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250103012303.746521-1-antonio.pastor@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>