ofp-actions: Make formatting and parsing functions take a struct argument.

ofp-actions: Make formatting and parsing functions take a struct argument.

An upcoming commit will add another parameter for parsing and formatting
actions.  It is much easier to add these parameters if they are
encapsulated in a struct, so this commit first makes that change.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Acked-by: Mark Michelson <mmichels@redhat.com>