LU-4788 lfsck: take ldlm lock before modifying visible object

Before the LFSCK modifying on the namespace visible object,
it needs to acquire related ibits lock to pervent the client
to cache stale information. The .lustre/lost+found/ and its
sub-directories also needs related ldlm locks.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I439e02e1b7b24e87e7e6e25c5084f1c98116e7f7
Reviewed-on: http://review.whamcloud.com/10986
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>