netdev-linux: Ref and unref the netdev_linux_cache_notifier for taps too.

netdev-linux: Ref and unref the netdev_linux_cache_notifier for taps too.

netdev-linux uses netdev_linux_cache_notifier to flush its cache when the
kernel notifies userspace that a particular network device's configuration
or status has changed.  This is as applicable to tap devices as to system
and internal devices, so we should create and destroy the notifier for
tap devices also.

I doubt that in practice it's possible to run ovs-vswitchd without having
a non-tap device open, at least with the kernel datapath, because the
local port for a bridge is not a tap device, so there should be no need to
backport this to older versions.

Reported-by: Gaetano Catalli <gaetano.catalli@gmail.com>