LU-15850 lmv: always space-balance r-r directories

LU-15850 lmv: always space-balance r-r directories

If the MDT free space is imbalanced, use QOS space balancing for
round-robin subdirectory creation, regardless of the depth
of the directory tree.  Otherwise, new subdirectories created
in parents with round-robin default layout may suddenly become
"sticky" on the parent MDT and upset the space balancing and
load distribution.

Add sanity/test_413h to check that round-robin dirs always balance.

Test-Parameters: testlist=sanity env=ONLY=413h,ONLY_REPEAT=100
Fixes: 38c4c538f5 ("LU-15216 lmv: improve MDT QOS space balance")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ia1d0b5b1a027cf14236f93ae34b5cf4929e76d23
Reviewed-on: https://review.whamcloud.com/47578
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>