ofp-actions, nx-match: Use mf_oxm_header() instead of explicit constants.

Following this change, only meta-flow.c uses any explicit NXM_* or OXM_*
constants.  An upcoming commit will actually remove the definitions of
these constants, hiding them behind a functional interface, for better
abstraction.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>