LU-14659 test: improve generate_uneven_mdts() in sanity.sh

LU-14659 test: improve generate_uneven_mdts() in sanity.sh

Improve generate_uneven_mdts() in several places:
1. set qos maxage to 1, so the result is up to date, and avoid filling
   up MDT.
2. fill MDT with files of size 64K other than 1M, so MDT imbalance is
   quicker to achieve.
3. when checking minimum imbalance after test, lookup max value from
   the result, other than by index stored before directory creation,
   because the result is dynamic if several MDTs have almost the same
   free space and inodes.

Lustre-change: https://review.whamcloud.com/44649
Lustre-commit: d45be79a069f527657c1ce91630183031ea42b27

Test-Parameter: trivial mdscount=2 mdtcount=4 testlist=sanity
Fixes: 233344d451e ("LU-13417 test: generate uneven MDTs early for sanity 413")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I2807101ff632404e25fdb640840d83d1991c88d9
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52751
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>