net: Remove redundant BUG_ON() check in phonet_pernet

Passing NULL to phonet_pernet causes a crash via BUG_ON.
Dereferencing net in net_generic() also has the same effect.
This patch removes the redundant BUG_ON check on the same parameter.
Signed-off-by: NXu Wang <vulab@iscas.ac.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/phonet/pn_dev.c

@@ -35,8 +35,6 @@ static unsigned int phonet_net_id __read_mostly;
 
 static struct phonet_net *phonet_pernet(struct net *net)
 {
-	BUG_ON(!net);
-
 	return net_generic(net, phonet_net_id);
 }