LU-5888 utils: limit max_sectors_kb tunable setting

LU-5888 utils: limit max_sectors_kb tunable setting

Properly limit the max_sectors_kb tunable if larger than 32MB, as
added in commit 73bd456e896 (http://review.whamcloud.com/12723).
It was using the old value in "buf" instead of the updated "newval".

Change write_file() to use a file descriptor directly instead of
streams, since it only writes a single value before closing again.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I43a5b4c9d0e45649e0666fd58634286de53ebbe5
Reviewed-on: http://review.whamcloud.com/13240
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>