net: core: re-segment oversized TCP GSO skbs

An skb which exceeds an egress device limit loses its GSO feature mask and
is segmented into individual packets, although the device can still offload
smaller TCP GSO skbs.  A BIG TCP hop which feeds a non-BIG TCP one pays
that on every flow.

An unencapsulated TCP GSO skb which exceeds gso_max_size or gso_max_segs is
re-segmented with a max_segs which keeps each output inside that limit.  An
encapsulated or frag-list skb, a GSO type the device cannot offload, a
max_segs which leaves room for a single MSS and an skb whose transport header
is unset or stale all keep today's segmentation.

This path emits a plain GSO skb, not a BIG TCP one: inet_gso_segment() and
ipv6_gso_segment() write each output's whole length into the 16-bit L3
length field, so the size limit is also capped at what that field can
express.

The helper runs on the skb handed to the driver and only from the
netif_needs_gso() branch: an skb the device takes as it is pays nothing, and
an over-limit one pays the TCP header read and the features recomputation.

Measured on a veth -> bridge -> TAP -> guest virtio-net path, with BIG TCP
enabled on the veth endpoints and left off in the guest, so the skbs which
the veth hop accepts have to be segmented before the TAP device.  A single
iperf3 TCP flow, six alternating runs per state (`-t 15 -O 5`, fixed CPU
affinity and port tuple).  The middle column is the same tree with the
re-segmentation disabled:

  protocol  no BIG TCP   mixed, no reseg  mixed, resegmented
  TCP/IPv4  51.550 Gbps  15.850 Gbps      52.617 Gbps
  TCP/IPv6  52.050 Gbps  15.783 Gbps      51.933 Gbps

Coefficient of variation for the two mixed columns was 0.48% and 0.82% for
IPv4 and 0.44% and 0.44% for IPv6.  A BIG TCP hop which feeds a 64 KiB hop
loses 69% of the throughput of a path which never enables BIG TCP at all;
re-segmentation recovers it, 3.3x over the existing segmentation path and
within noise of the no BIG TCP baseline.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Assisted-by: LLM
Signed-off-by: Wang Zhan <wang.zhan@smartx.com>
Link: https://patch.msgid.link/20261008100651.2534957-6-wang.zhan@smartx.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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