ofproto: Get enough headroom in fail-open when connected to controller.

Since commit 750638bb "ofproto: Avoid ofpbuf_clone() for OFPAT_CONTROLLER
common case," send_packet_in() needs at least DPIF_RECV_MSG_PADDING bytes
of headroom, which ofpbuf_clone() doesn't supply.  This commit should fix
that.

This fixes an assertion failure in ofpbuf_prealloc_headroom() via
send_packet_in().

Reported-by: Justin Pettit <jpettit@nicira.com>