wifi: renew dynamic IP configuration after roaming

There are some APs that require a DHCP transaction before allowing
other traffic. This is meant to improve security by preventing the use
of static addresses. Currently we don't renew DHCP after roaming to a
new AP and this can lead to broken connectivity with APs that
implement the check described above. Also, even if unlikely, the new
AP could be in a different layer 3 network and so the old address
could be no longer valid.

Renew dynamic IP configuration after we detect the supplicant decided
to roam to a new AP. Note that we only trigger a DHCP client restart;
the DHCP client already implements the logic to renew the previous
address and fall back to a full request in case of NAK or timeout.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/449

src/devices/wifi/nm-device-wifi.c

@@ -2564,6 +2564,15 @@ supplicant_iface_notify_current_bss (NMSupplicantInterface *iface,
 		       new_bssid ?: "(none)",
 		       (new_ssid_s = _nm_utils_ssid_to_string (new_ssid)));
 
+		if (new_bssid) {
+			/* The new AP could be in a different layer 3 network
+			 * and so the old DHCP lease could be no longer valid.
+			 * Also, some APs (e.g. Cisco) can be configured to drop
+			 * all traffic until DHCP completes. To support such
+			 * cases, renew the lease when roaming to a new AP. */
+			nm_device_update_dynamic_ip_setup (NM_DEVICE (self));
+		}
+
 		set_current_ap (self, new_ap, TRUE);
 	}
 }