LU-16036 test: make sanity-lfsck 15d more robust

LU-16036 test: make sanity-lfsck 15d more robust

Now migrating directory LFSCK is not fully supported, thus accessing
such directory may fail. To make sanity-lfsck 15d more robust,
reformat servers after test.

Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=sanity-lfsck env=ONLY=15d,ONLY_REPEAT=100
Fixes: 54a2d4662b58 ("LU-15868 lfsck: don't crash upon dir migration failure")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ie613737ab3e8d294e1f9e5709ceb35baa75790ad
Reviewed-on: https://review.whamcloud.com/48048
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>