mcserver: use pktsize variable rather than calculating it again

Makes the code shorter and less error prone

Change-Id: If6eca1b13458c14977464be0d940e98adb4b8764
Reviewed-on: http://review.couchbase.org/38290
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>