net: netpoll: extract core of netpoll_cleanup

Extract the core part of netpoll_cleanup(), so, it could be called from
a caller that has the rtnl lock already.

Netconsole uses this in a weird way right now:

	__netpoll_cleanup(&nt->np);
	spin_lock_irqsave(&target_list_lock, flags);
	netdev_put(nt->np.dev, &nt->np.dev_tracker);
	nt->np.dev = NULL;
	nt->enabled = false;

This will be replaced by do_netpoll_cleanup() as the locking situation
is overhauled.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>