LU-4788 lfsck: verify .lustre/lost+found at the LFSCK start

/ROOT/.lustre/lost+found/ is a special directory to hold the
objects that the LFSCK does not exactly know how to handle,
such as orphans. So before the LFSCK scanning the system,
the consistency of such directory needs to be verified firstly
to allow the users it during the LFSCK.

fid_seq_is_dot_lustre() is a duplication of fid_seq_is_dot(),
drop it and cleanup the code.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I95cac84bed1ae16c8c86e495db0120d964395b5e
Reviewed-on: http://review.whamcloud.com/10987
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>