connmgr: Move send_len from ofputil_packet_in to ofproto_packet_in.

send_len is not directly part of the OpenFlow packet_in message, at least
given that it is partially redundant with packet_len.  send_len is, rather,
a request to the connmgr that expresses how many bytes the action requested
be sent to the controller, but the connmgr cannot always honor it.

Signed-off-by: Ben Pfaff <blp@nicira.com>