LU-549 llite: Improve statfs performance if selinux is disabled

LU-549 llite: Improve statfs performance if selinux is disabled

Even if selinux is disabled, client still tries to get selinux
attributes from MDS. As xattrs are not yet cached, this significantly
slows down xattr heavy operations like ls -l. This patch forces
to return -EOPNOTSUPP on the client side if selinux is disabled.
It speeds up ls -l 25% for cold-cache case and 50% for hot-cache
case.

Signed-off-by: Yevheniy Demchenko <zheka@uvt.cz>
Change-Id: I5e416093bba4126e5fcad62d8c0a2963c1866386
Reviewed-on: http://review.whamcloud.com/2503
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>