MB-4786: actually exclude page cache size from used memory size

Previous commit unly did that for v1 output from sigar_port

Change-Id: I5771e748fd548e0ea9324341ba63493c84083629
Reviewed-on: http://review.couchbase.org/27722
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>