cifs: Send witness register and unregister commands to userspace daemon

+ Define the generic netlink family commands and message attributes to
  communicate with the userspace daemon

+ The register and unregister commands are sent when connecting or
  disconnecting a tree. The witness registration keeps a pointer to
  the tcon and has the same lifetime.

+ Each registration has an id allocated by an IDR. This id is sent to the
  userspace daemon in the register command, and will be included in the
  notification messages from the userspace daemon to retrieve from the
  IDR the matching registration.

+ The authentication information is bundled in the register message.
  If kerberos is used the message just carries a flag.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>