meta-flow: Remove cmap dependency.

meta-flow: Remove cmap dependency.

Previous patch 04f48a68 ("ofp-actions: Fix variable length meta-flow OXMs.")
introduced dependency of an internal library (cmap.h) to ovs public
interface (meta-flow.h) that may cause potential building problem. In this
patch, we remove cmap from struct mf_field, and provide a wrapper struct
vl_mff_map that resolve the dependency problem.

Fixes: 04f48a68c428 ("ofp-actions: Fix variable length meta-flow OXMs.")
Suggested-by: Joe Stringer <joe@ovn.org>
Suggested-by: Daniele Di Proietto <diproiettod@vmware.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>