Previously flow dissectors were referenced without first checking that
they are in use and correctly populated by TC. This patch fixes this by
checking each flow dissector key before referencing them.

Fixes: 5571e8c9 ("nfp: extend flower matching capabilities")
Signed-off-by: NPieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>