glusterd/snapshot: umount2 on OSX/NetBSD is unmount

Change-Id: I8de4d47bb2a54b915243ea029cce2585fba34876
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7651
Reviewed-by: Justin Clift <justin@gluster.org>
Tested-by: Justin Clift <justin@gluster.org>
Reviewed-by: Anand Avati <avati@redhat.com>