LU-3180 tests: lfsck.sh improved messages

LU-3180 tests: lfsck.sh improved messages

Use skip_env() for the SHARED_DIRECTORY check, so that lfsck is not
skipped for the normal test environment (VMs sharing a single host)
or single-node tests.

Add some more information about lfsck.sh corruption, for improved
debugging in case of problems.

Test-Parameters: testlist=lfsck
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I56d7a52b0f880587d988c842fb6ee0b14b699ca2
Reviewed-on: http://review.whamcloud.com/6206
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>