ofproto: Report multipart buffer overflow.

ofproto: Report multipart buffer overflow.

Because our size of the multipart request buffer is effectively 0,
always report multipart buffer overflow error for multipart requests
with "more" flag set.

While this might be simpler if done in ofp-msgs.c, i put this in
ofproto.c because it seems like a better place to implement the
buffering logic if/when desirable.

An option question: what to do for the following messages with the
same xid?  especially for the last message which doesn't have the
"more" flag set?  it would be neater to detect and drop them.

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