LU-14807 lfsck: fix race in lfsck_pos_fill

LU-14807 lfsck: fix race in lfsck_pos_fill

There is a race for lfsck->li_di_dir between lfsck_di_dir_put and
lfsck_pos_fill, which could cause lfsck_pos_fill to use freed
lfsck->li_di_dir (struct osd_it_ea) and trigger GPF.

Change-Id: Iedadf03ac15d128bb051aea8aafa24dbcd2704fb
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44130
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>