netbuf: Fix curblocks/maxblocks

We previously didn't use these variables resulting in high amounts of
malloc/free.

Change-Id: Iaeae2f462d93707f3baec286c6231529d191c95b
Reviewed-on: http://review.couchbase.org/38925
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>