smb2_server: add and use a function that calculated the remaining channels

smb2_server: add and use a function that calculated the remaining channels

This is useful for debugging, but also simplies the following changes,
where client->connections may hold disconnected connections until
all pending requests are finished.

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