ksmbd: prevent rename with empty string

Client can send empty newname string to ksmbd server.
It will cause a kernel oops from d_alloc.
This patch return the error when attempting to rename
a file or directory with an empty new name string.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>