batman-adv: delete global entry in case of roaming

When receiving a DEL change for a client due to a roaming event (change is
marked with TT_CLIENT_ROAM), each node  has to check if the client roamed
to itself or somewhere else.

In the latter case the global entry is kept to avoid having no route at all
otherwise we can safely delete the global entry

Signed-off-by: Antonio Quartulli <ordex@autistici.org>