cifs: add missing return value check for cifs_sb_tlink

Whenever a tlink is obtained by cifs_sb_tlink, we need
to check that the tlink returned is not an error.
It was missing with the last change here.

Fixes: b3edef6b ("cifs: allow dumping keys for directories too")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>