nfp: add helper to get flower cmsg length

Add a helper function that returns the length of the cmsg data when given
the cmsg skb

Signed-off-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>