LU-17171 test: improve sanity-quota test_41

LU-17171 test: improve sanity-quota test_41

On zfs backend, df result of project quota may print be a slightly
larger block used than quota result because the former is calcuated
in filesystem block size which is 4K.

Update sanity-quota test_41 to make it more robust.

Test-Parameters: trivial testlist=sanity-quota fstype=zfs
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ide51d9aaeb8907eb77acc30fa4fc76dcc16e8de0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52591
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>