LU-18310 tests: add debugging to test_metabench

LU-18310 tests: add debugging to test_metabench

Both parallel-scale and parallel-scale-nfs are intermittently
failing test_metabench with "No space left on device" (ENOSPC)
or "Disk quota exceeded" (EDQUOT), even though this test is
creating only about 10-20k files.

Add some debugging to see where all of the space has gone, and
what quota limits are being set.  It may be that some earlier
test (e.g. compilebench) is leaving too much junk behind.

The failure rate is very low (only 2/637 runs in the past 4 weeks),
so it likely needs to be landed to catch a failure.

Test-Parameters: trivial testlist=parallel-scale
Test-Parameters: testlist=parallel-scale-nfsv4
Test-Parameters: testgroup=full-part-1
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie35ae677032ccc8113cbad5dc5a7b0504149717f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56582
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Deiter <adeiter@ddn.com>
Reviewed-by: Elena <elena.gryaznova@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>