LU-5831 lfsck: extend lfsck_request::lr_pool_name

LU-5831 lfsck: extend lfsck_request::lr_pool_name

Fix some issues found during Lustre source static analysis:

1) Extend lfsck_request::lr_pool_name size to match the
   lmv_mds_md_v1::lmv_pool_name.
2) Check lfsck->li_obj_dir inside lfsck_close_dir() before using.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I84443089135c5de1b9fa89eb76e5cd623412a01f
Reviewed-on: http://review.whamcloud.com/12534
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>