ofp-actions: Return action size

Modify ofpacts_put_openflow11_actions() to return the length of actions
appended. This will be used when encoding Packet Out messages for
Open Flow 1.1 and 1.2. The motivation for this is to avoid open coding
the size calculation which may end up being needed elsewhere too.

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