LU-762 ldiskfs: don't drop directory nlink to 0

LU-762 ldiskfs: don't drop directory nlink to 0

When landing the nlink patch for ext4, for an unknown reason the
logic in ext4_dec_count() was changed from the ext3 version of
the patch. It now drops the nlink = 0 temporarily and then
if it is a directory with nlink == 0 it increases nlink again.

Instead, only drop nlink if it is larger than 2.

Change-Id: Ieeff3e45daea56f502848f9c2b0fb04f0a9d2b6d
Author: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Yang Sheng <ys@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1644
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>