LU-13636 obdclass: drop nlink if directory is removed

LU-13636 obdclass: drop nlink if directory is removed

To make e2fsck happy.  Otherwise, all the features using
local directories (quota, nodemap, nid tables) can leave
orphaned objects as nlink doesn't drop to 0.

Lustre-change: https://review.whamcloud.com/38844
Lustre-commit: c6d5c6606a38e2b550a81591935b0091faba4a2e

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I9e20a304d66c61f312168715e888757bc06b6ed0
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/44466
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>