dhcpv4: fix ubus events

dhcpv4: fix ubus events

Note that req->ciaddr which was used to generate the broadcast message is
completely under client control and isn't checked, meaning that a
buggy/malicious client could cause broadcast messages containing an arbitrary
IP address.

While addressing this, move the broadcast message generation into
dhcpv4_lease(), so that the function can release the assignment straight away
and return NULL (it seems to return the released assignment just so that it can
be broadcast and later reaped by the expiry timer).

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/270
Signed-off-by: Álvaro Fernández Rojas <[email protected]>