When we get a GSO packet from an untrusted source, we need to
ensure that it is sufficiently long so that we don't end up
crashing.

Based on discovery and patch by Ian Campbell.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Tested-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>