tty: n_gsm: improper skb_pull() use was leaking framed data

gsm_dlci_data_output_framed() was doing:
memcpy(dp, skb_pull(dlci->skb, len), len);

The problem is skb_pull() returns the post-increment data ptr
so the first chunk of dlci->skb->data is leaked.
Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/n_gsm.c

@@ -875,7 +875,8 @@ static int gsm_dlci_data_output_framed(struct gsm_mux *gsm,
 		*dp++ = last << 7 | first << 6 | 1;	/* EA */
 		len--;
 	}
-	memcpy(dp, skb_pull(dlci->skb, len), len);
+	memcpy(dp, dlci->skb->data, len);
+	skb_pull(dlci->skb, len);
 	__gsm_data_queue(dlci, msg);
 	if (last)
 		dlci->skb = NULL;