ofp-actions: Make ofpact_finish() harder to misuse.

ofp-actions: Make ofpact_finish() harder to misuse.

It's pretty easy to forget to update the pointer to an ofpact when
finishing it.  This commit forces the caller to pass a pointer-to-pointer
instead, and uses that to automatically update the pointer.  There still
could be cases that retain other pointers into the ofpbuf, but I imagine
that this is harder to misuse.

Suggested-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>