MB-11259: Buffer acknowledgment should have extlen of 4

Change-Id: Ic020e7260db1138f607fb5b3e068f9955f3e5c58
Reviewed-on: http://review.couchbase.org/38790
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>