LU-5473 tests: print space usage in sanity test_51b

In sanity.sh test_51b print out the space usage before and after
the test so that the failure can be debugged.

Skip test_51b and test_51ba for ZFS when running regular review
tests, since there isn't a limit of 60000 subdirectories (ZFS
nlink is a 64-bit number), and they take a long time to run in
a VM (20 minutes combined).

Test-Parameters: alwaysuploadlogs \
envdefinitions=SLOW=yes,ENABLE_QUOTA=yes,ONLY=51 \
mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs \
clientdistro=el6 ossdistro=el6 mdsdistro=el6 \
mdtcount=1 mdssizegb=2 ostcount=2 ostsizegb=8 \
testlist=sanity

Test-Parameters: alwaysuploadlogs \
envdefinitions=SLOW=yes,ENABLE_QUOTA=yes,ONLY=51 \
mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs \
clientdistro=el6 ossdistro=el6 mdsdistro=el6 \
mdtcount=1 mdssizegb=2 ostcount=2 ostsizegb=8 nettypes=o2ib \
testlist=sanity

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I21b072fbcb05dea3fd7803bf3353de11ffbcab07
Reviewed-on: http://review.whamcloud.com/12185
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>