LU-2489 mdc: Restrict access to hsm_nl file

LU-2489 mdc: Restrict access to hsm_nl file

Disallow ordinary users from writing to /proc/fs/lustre/mdc/*/hsm_nl
to prevent denials of service.

Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Change-Id: Ibd37cb9365bc4583fa964b18c3d87f264d1ca33d
Reviewed-on: http://review.whamcloud.com/4825
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>