mcreq_dup_packet: Fix bad iov handling

We're using 'iov' as the array offset, and then treating the passed IOV
itself as an array. We should be treating it as a lone pointer.

Change-Id: I0be96676d59d2530c527943b4a28e7330d112599
Reviewed-on: http://review.couchbase.org/38189
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>