nfp: flower: ensure dead neighbour entries are not offloaded

Previously only the neighbour state was checked to decide if an offloaded
entry should be removed. However, there can be situations when the entry
is dead but still marked as valid. This can lead to dead entries not
being removed from fw tables or even incorrect data being added.

Check the entry dead bit before deciding if it should be added to or
removed from fw neighbour tables.

Fixes: 8e6a9046 ("nfp: flower vxlan neighbour offload")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>