LU-9202 lfsck: skip unavailable targets when sync failures

LU-9202 lfsck: skip unavailable targets when sync failures

It is normal that some target (MDT or OST) may become unavailable
when the LFSCK engine tries to sync failures with related target.
Under such case, just skip such target without LBUG.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ib677e6fec121e946caafb34ebc71b8b3068bd6f5
Reviewed-on: https://review.whamcloud.com/25931
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>