LU-7429 tests: generate dangling name entry properly

When start LFSCK, before scanning the system, it needs to create
the objects ".lustre/lost+found/MDTxxxx" to hold orphans if they
do not exist. Such creation may re-use the backend object (inode
for ldiskfs backend) that was original referenced by the dangling
name entry. If such case happened, the dangling failure injection
will fail, then the LFSCK will not find dangling name entry.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I3c8be2e3f83e10a454bbc6175242d0a982211d87
Reviewed-on: http://review.whamcloud.com/21412
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>