LU-12029 utils: disable max_sectors_kb autotuning

LU-12029 utils: disable max_sectors_kb autotuning

Disable automatic max_sectors_kb tuning via mount.lustre by default.
This conflicts in EXA with tune_devices.sh that has SFA-specific
tuning parameters from the tune_devices.sh script.

Disable l_tunedisk in /etc/udev/rules.d/99-lustre-server.rules by
default for EXA so that it does not conflict with the EXA script.

Allow shrinking max_sectors_kb if explicitly set via mount option.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I58cf548d08f8680ec5d6ffd00e936a5d903ebbe5
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/41839
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>