gigaset: linearize skb

The code of the Gigaset driver assumes that sk_buff-s coming
from the ISDN4Linux subsystem are always linear. Explicitly
calling skb_linearize() is cheap if they are, but much more
robust in case they ever aren't.

Impact: robustness improvement
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/gigaset/i4l.c

@@ -51,6 +51,12 @@ static int writebuf_from_LL(int driverID, int channel, int ack,
 		return -ENODEV;
 	}
 	bcs = &cs->bcs[channel];
+
+	/* can only handle linear sk_buffs */
+	if (skb_linearize(skb) < 0) {
+		dev_err(cs->dev, "%s: skb_linearize failed\n", __func__);
+		return -ENOMEM;
+	}
 	len = skb->len;
 
 	gig_dbg(DEBUG_LLDATA,