cifs: Add handling of blank password option

The option to have a blank "pass=" already exists, and with
a password specified both "pass=%s" and "password=%s" are supported.
Also, both blank "user=" and "username=" are supported, making
"password=" the odd man out.

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>