device: commit MTU during stage2

Currently we commit the MTU to the device when updating the IP
configuration, or when a port device is added to the controller. This
means that for a connection with DHCP, the MTU is set only after DHCP
has completed. In particular, if DHCP doesn't complete and the
connection has an infinite timeout, the MTU is never set.

_commit_mtu() tracks different sources for the MTU of a device, and
each source has a different priority. Among these sources there are
the parent link (for VLANs), a dynamic IP configuration (DHCP, PPP)
and the connection profile.

A MTU from the connection always has the highest priority and
overrides other sources.

Therefore, if the connection specifies an MTU it can be applied at
stage2, even before configuring IP addressing.

https://bugzilla.redhat.com/show_bug.cgi?id=1890234
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/859

src/core/devices/nm-device.c

@@ -8468,6 +8468,8 @@ activate_stage2_device_config(NMDevice *self)
 
     lldp_setup(self, NM_TERNARY_DEFAULT);
 
+    _commit_mtu(self, NULL);
+
     nm_device_activate_schedule_stage3_ip_config_start(self);
 }