drivers/isdn/i4l/isdn_common.c fix small resource leak

Coverity CID: 1356 RESOURCE_LEAK

I found a very old patch for this that was Acked but did not get applied
https://lists.linux-foundation.org/pipermail/kernel-janitors/2006-September/016362.html

There looks to be a small leak in isdn_writebuf_stub() in isdn_common.c, when
copy_from_user() returns an un-copied data length (length != 0).  The below
patch should be a minimally invasive fix.

Signed-off-by: Darren Jenkins <darrenrjenkins@gmailcom>
Acked-by: Karsten Keil <kkeil@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/i4l/isdn_common.c

@@ -1977,8 +1977,10 @@ isdn_writebuf_stub(int drvidx, int chan, const u_char __user * buf, int len)
 	if (!skb)
 		return -ENOMEM;
 	skb_reserve(skb, hl);
-	if (copy_from_user(skb_put(skb, len), buf, len))
+	if (copy_from_user(skb_put(skb, len), buf, len)) {
+		dev_kfree_skb(skb);
 		return -EFAULT;
+	}
 	ret = dev->drv[drvidx]->interface->writebuf_skb(drvidx, chan, 1, skb);
 	if (ret <= 0)
 		dev_kfree_skb(skb);