vxlan: Allow changing ageing time

In a similar fashion to the bridge device, allow changing the ageing
time of the VxLAN device by scheduling its timer to fire if the ageing
time changed.

One use case is selftests where learning / ageing of VxLAN FDB entries
is tested. The default ageing time is 5 minutes, which is too long for a
simple selftest.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>