LU-1519 llite: Add a mount flag to clarify DAC handling

LU-1519 llite: Add a mount flag to clarify DAC handling

Add a mount flag to control use of OBD_IOC_FID2PATH by processes
without CFS_CAP_DAC_READ_SEARCH.

Change-Id: Ibbca25ad511d80e245716d27fca4287979667db6
Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Reviewed-on: http://review.whamcloud.com/3104
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Keith Mannthey <keith@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>