dup_packet: readjust body len after inflation

Change-Id: I00b70f9bfb7c4ed27dd975c4154805b383a8b91b
Reviewed-on: http://review.couchbase.org/37782
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>