cifs: Use kzfree() to zero out the password

It's safer to zero out the password so that it can never be disclosed.

Fixes: 0c219f5799c7 ("cifs: set domainName when a domain-key is used in multiuser")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>