s3:smbd: add smbd_server_disconnect_client[_ex]()

s3:smbd: add smbd_server_disconnect_client[_ex]()

With multichannel things may not happen only on one connection.
We may need to disconnect all connections of a client, when something
bad happens.

The first users of this will be the lease/oplock break code,
if they are not able allocate memory or something similar
we need to bail out.

Having a special smbXsrv_client based function is better than
calling exit_server*() directly.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11897

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>