LU-5888 utils: limit max_sectors_kb tunable setting

Limit the value set by mount.lustre set_blockdev_tunables() to a
reasonable 32MB instead of the maximum possible amount, since the
parsing of max_hw_sectors_kb might be bad, or it just returns a
value much larger than we need.

Also quiet the printing of the max_sectors_kb tunable that was added
in commit 9813961151e (http://review.whamcloud.com/9865) so that it
only prints something when the value is actually changed, instead of
printing it for every tunable even if the value is the same.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I648c2d8484ae5cef59ab62421cd01bc0ed02fcd6
Reviewed-on: http://review.whamcloud.com/12723
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Blake Caldwell <blakec@ornl.gov>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>