nfp: flower: ensure TCP flags can be placed in IPv6 frame

Previously we did not ensure tcp flags have a place to be stored
when using IPv6. We correct this by including IPv6 key layer when
we match tcp flags and the IPv6 key layer has not been included
already.

Fixes: 07e1671c ("nfp: flower: refactor shared ip header in match offload")
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>