ofp-msgs: ofpmp_reserve() must add "more" flag when splitting stats.

ofp-msgs: ofpmp_reserve() must add "more" flag when splitting stats.

Commit 982697a4d2 "ofp-msgs: New approach to encoding and decoding OpenFlow
headers." introduced a bug in multipart message composition such that, when
a multipart message actually requires multiple parts, the OFPSF_REPLY_MORE
bit failed to be set on any of the parts, which in turn caused the receiver
to see a truncated reply.  This commit fixes the problem.

This commit also adds a test case that would have detected the problem and
fixes a minor bug in that test case (if an error actually occurred, then
the test case would hang).

Bug #12766.
Reported-by: James Schmidt <jschmidt@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>