nfp: flower: fix ABI mismatch between driver and firmware

Fix an issue where the driver wrongly detected ipv6 neighbour updates
from the NFP as corrupt. Add a reserved field on the kernel side so
it is similar to the ipv4 version of the struct and has space for the
extra bytes from the card.

Fixes: 9ea9bfa1 ("nfp: flower: support ipv6 tunnel keep-alive messages from fw")
Signed-off-by: Louis Peens <louis.peens@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>