ofp-util: Correct setting of Flow Mod cookie on encode

ofp-util: Correct setting of Flow Mod cookie on encode

When the command of a Flow Mod messages is ADD the cookie should be set as
fm->new_cookie, this is the new cookie value to be set. Otherwise it
should be set as fm->cookie, internally this is the cookie value to match.

Also remove 'XXX' marker from the matching code in the decoder.
I am now comfortable with the implementation.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>