l2tp: avoid skb truesize bug if headroom is increased

A user reported seeing occasional bugs such as the following when
using the L2TP driver.

  SKB BUG: Invalid truesize (272) len=72, sizeof(sk_buff)=208

When L2TP adds its header in the transmit path, it might need to
increase the headroom of the skb. In some cases, the increased
headroom trips a kernel bug when the skb is freed because the skb has
grown beyond its truesize value. The fix is to increase the truesize
by the amount of headroom added, after orphaning the skb.

While here, fix a misleading comment.

Thanks to Iouri Kharon <bc-info@styx.cabel.net> for the initial
report and testing the fix.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>