device: skip VLAN chain lookup for existing kernel interfaces

[ghoker143]

netifd's __device_get() treats any interface name containing a '.' as a
VLAN-style device by immediately dispatching to get_vlan_device_chain().

Modern externally-managed interfaces such as the rmnet mux links created
by ModemManager for QMI QMAP multiplexing (e.g. qmapmux0.0, qmapmux0.1)
are legitimate kernel netdevices that happen to contain a dot. When
ModemManager calls notify_proto with such an ifname, netifd incorrectly
tries to resolve it as a VLAN device:

ubus call network.device status '{"name":"qmapmux0.0"}'
=> { "type": "VLAN", "present": false }

This causes notify_proto to fail with "Unknown error" and the interface
never comes up under netifd control.

Fix: before dispatching to get_vlan_device_chain(), probe the kernel with
if_nametoindex(). If the full dotted name already exists as a real netdev,
skip the VLAN path and fall through to the normal device lookup / creation
path. This preserves all existing VLAN behaviour: when eth0.10 does not
yet exist in the kernel, if_nametoindex("eth0.10") returns 0 and netifd
still creates it as a VLAN device as before.