net: lan743x: fix RX checksum use-after-free

lan743x_rx_process_buffer() adds each non-first receive buffer to the
head skb's frag_list.  On the last descriptor, lan743x_rx_trim_skb()
linearizes the head and frees the fragment skb metadata.

The checksum-success path then writes ip_summed through the local skb
pointer, which still points to the final fragment.  This causes a
use-after-free write when a packet spans more than one receive buffer.

Set ip_summed on the surviving head skb instead.  Multi-buffer receive
can occur after a live MTU increase because existing ring entries keep
their old buffer size until they are replenished.

A KUnit test invoking lan743x_rx_process_buffer() with a two-buffer
packet produced a one-byte KASAN use-after-free write before this change.
The same test passed after the change.  The driver object also builds
with W=1.  This was not tested on physical LAN743x hardware.

Fixes: cd6910501cfd ("net: lan743x: Add support for Rx IP & TCP checksum offload")
Cc: stable@vger.kernel.org
Signed-off-by: Mark Amirkan <markdamirkan@gmail.com>
Reviewed-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
Link: https://patch.msgid.link/20260913-b4-send-lan743x-uaf-v1-1-73d563d08ba9@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>