glusterd: Added validation function for performance cache max and min size.

Change-Id: I0b8dbc4b65412b8aff24873f030c03e3dcfcb988
BUG: 782095
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4541
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>