LUDOC-496 nrs: Format of TBF rule name field is not defined

LUDOC-496 nrs: Format of TBF rule name field is not defined

Lustre manual should define the format (alphanumeric + '_') and length
(15 characters) of a TBF rule name field.

This is important because there is buffer overflow for the TBF rule name
(LU-15059): the length is not checked by ptlrpc.

Change-Id: I29efb7e5891c0fba308b683bf92b26e4fefa7695
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/45144
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>