LU-9132 utils: tuning max_sectors_kb on mount

LU-9132 utils: tuning max_sectors_kb on mount

Sometimes user doesn't want the max_sectors_kb being
overwritten on MDT/OST mount, we should provide a way for
user to have more control over this important parameter,
a new mount option 'max_sectors_kb' is introduced:

- When max_sectors_kb isn't specified on mount, change the
  max_sectors_kb to max_hw_sectors_kb, it's default behavior
  suited for most users;
- When max_sectors_kb is specified as zero, leave the old
  setting of max_sectors_kb untouched;
- When max_sectors_kb is specified as a positive number,
  change the max_sectors_kb to this number arbitrarily;

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I251d75bbdf16d8cb6d503bbbfc69fb18993f7a3e
Reviewed-on: https://review.whamcloud.com/25483
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>