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>
Merge remote-tracking branch 'membase/3.0'

* membase/3.0:
  MB-11259: Buffer acknowledgment should have extlen of 4

Change-Id: Ia9e69377b1f2e970bbc331bcd110e42c299bb686