LU-12295 mdd: don't LBUG() if dir nlink is wrong

LU-12295 mdd: don't LBUG() if dir nlink is wrong

Sometimes dir nlink may not be correctly decreased: subdir is remote,
when it's unlinked, its dirent is removed, but parent nlink decrease
failed.

Don't assert this in osd_destroy(), but print an error message and
continue since we've checked directory is empty.

Add OBD_FAIL_OSD_REF_DEL to simulate the error above.

Add sanity 48f.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I483aaf7a62b7761868b5e2af8dbfa92929fda78c
Reviewed-on: https://review.whamcloud.com/39734
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>