bluetooth: Fix use-before-initiailized var.

net/bluetooth/l2cap_core.c: In function ‘l2cap_recv_frame’:
net/bluetooth/l2cap_core.c:3612:15: warning: ‘sk’ may be used uninitialized in this function
net/bluetooth/l2cap_core.c:3612:15: note: ‘sk’ was declared here

Actually the problem is in the inline function l2cap_data_channel(), we
branch to the label 'done' which tests 'sk' before we set it to anything.

Initialize it to NULL to fix this.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bluetooth/l2cap_core.c

@@ -3609,7 +3609,7 @@ static int l2cap_ertm_data_rcv(struct sock *sk, struct sk_buff *skb)
 static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk_buff *skb)
 {
 	struct l2cap_chan *chan;
-	struct sock *sk;
+	struct sock *sk = NULL;
 	struct l2cap_pinfo *pi;
 	u16 control;
 	u8 tx_seq;