ofproto: Add pipeline fields support for OF 1.5 packet-out

ofproto: Add pipeline fields support for OF 1.5 packet-out

This patch decodes pipeline fields from a packet-out message, and populates
the pipeline fields into datapath. Error OFPERR_OFPBRC_PIPELINE_FIELDS_ONLY
is returned if the match field of a packet-out messages contains any
non pipeline fields. Currently, the supported pipeline fields
are as following.

* metadata fields:
    - in_port, in_port_oxm

* tunnel fields:
    - tun_id, tun_src, tun_dst, tun_ipv6_src, tun_ipv6_dst
    - tun_gbp_id, tun_gpb_flags, tun_flags
    - tun_metadata0 - tun_metadata63

* register fields:
    - metadata
    - reg0 - reg-15, xreg0 - xreg7, xxreg0 - xxreg3

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>