batman-adv: in case of roaming mark the client with TT_CLIENT_ROAM

In case of a client roaming from node A to node B, the latter have to mark the
corresponding global entry with TT_CLIENT_ROAM (instead of TT_CLIENT_PENDING).

Marking a global entry with TT_CLIENT_PENDING will end up in keeping such entry
forever (because this flag is only meant to be used with local entries and it is
never checked on global ones).

In the worst case (all the clients roaming to the same node A) the local and the
global table will contain exactly the same clients. Batman-adv will continue to
work, but the memory usage is duplicated.

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