EX-852 ldiskfs: adjust default block allocation threshold

EX-852 ldiskfs: adjust default block allocation threshold

Use default 25% 15% might be too aggressive, especially
with DDN OST will be 1PB soon, let's adjust it to 15% and 10%,
also limit the max space to 15TB,10TB

Change-Id: I472160fc37fa8d119f13084f00bb22ea55c5ca18
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/41835
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>