ppp: linearize and segment bridged packets when required

PPP channel bridging hands packets received on one channel directly to
the start_xmit() of another. Packets received by PPPoE can be
non-linear, GSO since commit 55a5d8fca836 ("net: pppoe: implement
GRO/GSO support"), or CHECKSUM_PARTIAL when injected with a
virtio_net_hdr. The regular transmit path never sees such packets, and
only PPPoE, which uses dev_queue_xmit(), handles them:

- PPP Async reads the whole packet through skb->data, transmitting
  unrelated memory for non-linear packets.
- PPP Async and Sync send GSO packets as single frames of up to 64KB,
  and CHECKSUM_PARTIAL packets with a bad L4 checksum.
- PPTP and L2TP cannot segment a TCP GSO packet once their GRE or UDP
  header is pushed, and do not finish pending checksums.

Add a xmit_offload flag to struct ppp_channel, set for PPPoE. For other
channels, segment GSO packets in software, using the PPP protocol field
as the L2 header, finish CHECKSUM_PARTIAL checksums (including SCTP
CRC32c), and linearize packets for channels without direct_xmit, as
ppp_push() does. Packets are dropped if any step fails, or if the
channel is busy, as before.

skb->dev is NULL at this point for PPPoE (cleared by __sk_receive_skb()),
and tunnel and IPv6 UFO GSO handlers dereference it: only TCP, UDP L4
and fraglist GSO packets without encapsulation are segmented, others
are dropped.

Receive paths like L2TP pull headers without updating the checksum
state, possibly leaving a CHECKSUM_PARTIAL state with csum_start before
skb->data. Reset it to CHECKSUM_NONE, as skb_postpull_rcsum() does, so
that skb_checksum_help() does not warn, and so that PPPoE does not
offload a checksum at a wrong offset.

Tested in a VM by bridging PPPoE (on a TAP with IFF_NAPI_FRAGS, or on
veth) to PPP Async on a PTY, PPPoL2TP and PPPoE: non-linear packets no
longer leak garbage, GSO packets (including fraglist GRO) are sent as
valid segments for IPv4 and IPv6, CHECKSUM_PARTIAL packets get a valid
checksum, and PPPoE still receives GSO packets unchanged.

Fixes: 4cf476ced45d ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls")
Fixes: 55a5d8fca836 ("net: pppoe: implement GRO/GSO support")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260908072135.877364-1-qingfang.deng@linux.dev
Co-developed-by: Qingfang Deng <qingfang.deng@linux.dev>
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20261026155927.707655-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>