ofp-actions: Make composing actions harder to screw up.

ofp-actions: Make composing actions harder to screw up.

Until now, composing a fixed-length action with ofpact_put_<NAME>() failed
to append any padding required after the action.  This commit changes that
so that these calls now add padding.  This meant that the function
ofpact_pad(), which was until now required in various unintuitive places,
is no longer required, and removes it.

Variable-length actions still require calling ofpact_update_len() after
composition.  I don't see a way to avoid that.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>