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.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I9e20a304d66c61f312168715e888757bc06b6ed0
Reviewed-on: https://review.whamcloud.com/38844
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>