MB-11259: Buffer acknowledgment should have extlen of 4

Change-Id: I2b22316e876a81211fe1cf676cb4f8c531162145
Reviewed-on: http://review.couchbase.org/37764
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>