batman-adv: properly store the roaming time

in case of a new global entry added because of roaming, the roam_at field must
be properly initiated with the current time. This value will be later use to
purge this entry out on time out (if nobody claims it). Instead roam_at field
is now set to zero in this situation leading to an immediate purging of the
related entry.

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