skb_headroom() is unsigned so "skb_headroom(skb) + toff" is also
unsigned and can't be less than zero.  This test was added in 66d50d25:
"u32: negative offset fix"  It was supposed to fix a regression.
Signed-off-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>