Use the metadata disk size stat for replica vbucket.

Change-Id: Ied887dd0ac0e3ac0ed309c554d27d95775c9874b
Reviewed-on: http://review.couchbase.org/40158
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>