fs/smb/client: Reset password pointer to NULL

Forget to reset ctx->password to NULL will lead to bug like double free

Cc: stable@vger.kernel.org
Cc: Willy Tarreau <w@1wt.eu>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Quang Le <quanglex97@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>