ofproto: Avoid buffer copy in OFPT_PACKET_IN path.

ofproto: Avoid buffer copy in OFPT_PACKET_IN path.

When a dpif passes an odp_msg down to ofproto, and ofproto transforms it
into an ofp_packet_in to send to the controller, until now this always
involved a full copy of the packet inside ofproto.  This commit eliminates
this copy by ensuring that there is always enough headroom in the ofpbuf
that holds the odp_msg to replace it by an ofp_packet_in in-place.

From Jean Tourrilhes <jt@hpl.hp.com>, with some revisions.