LU-11654 mdd: check ucred before using it

LU-11654 mdd: check ucred before using it

On the code path of client eviction, lu_ucred in mdd_close() is not
initialized so we need to check it before trying to dereference it.

Signed-off-by: Jinshan Xiong <jinshan.xiong@uber.com>
Change-Id: I51b2a30e651761d66470fedf39da8e99b47ea6b4
Reviewed-on: https://review.whamcloud.com/33638
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>