staging/lustre: fix 'program hangs' errors

Fix 'program hangs' defects found by Coverity version 6.5.1:
Missing unlock (LOCK)
Returning without unlocking.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3054
Lustre-change: http://review.whamcloud.com/5870


Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>