LU-14762 lmv: compare space to mkdir on parent MDT

LU-14762 lmv: compare space to mkdir on parent MDT

In QOS subdirectory creation, subdirectories are kept on parent MDT
if it is less full than average, however it checks weight other than
free space, while "weight = free space - penalty", if MDTs have
different penalties, the result is not accurate, therefore this may
not work.

Check free space instead, and loosen the critirion to allow the
free space within the range of QOS threshold.

Fixes: 3f6fc483013d ("LU-13439 lmv: qos stay on current MDT if less full")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Id34cf8f3f58fee9d329f0d05c2f7a6463b67dfe1
Reviewed-on: https://review.whamcloud.com/43997
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>