net: gro: fix napi_gro_cb zeroed alignment

Add 2 byte padding to napi_gro_cb struct to ensure zeroed member is
aligned after flush_id member was removed in the original commit.

Fixes: 4b0ebbca

 ("net: gro: move L3 flush checks to tcp_gro_receive and udp_gro_receive_segment")
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Link: https://lore.kernel.org/r/fca08735-c245-49e5-af72-82900634f144@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>