There doesn't appear to be a reason to truncate the allocation used for
flow_info, so do a full allocation and remove the unused empty struct.
GCC does not like having a reference to an object that has been
partially allocated, as bounds checking may become impossible when
such an object is passed to other code. Seen with GCC 13:

../drivers/net/ethernet/mediatek/mtk_ppe.c: In function 'mtk_foe_entry_commit_subflow':
../drivers/net/ethernet/mediatek/mtk_ppe.c:623:18: warning: array subscript 'struct mtk_flow_entry[0]' is partly outside array bounds of 'unsigned char[48]' [-Warray-bounds=]
  623 |         flow_info->l2_data.base_flow = entry;
      |                  ^~

Cc: Felix Fietkau <nbd@nbd.name>
Cc: John Crispin <john@phrozen.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: Mark Lee <Mark-MC.Lee@mediatek.com>
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: netdev@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230127223853.never.014-kees@kernel.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>