gro: Only reset frag0 when skb can be pulled, CVE-2011-2723

gro: Only reset frag0 when skb can be pulled, CVE-2011-2723

Currently skb_gro_header_slow unconditionally resets frag0 and
frag0_len.  However, when we can't pull on the skb this leaves
the GRO fields in an inconsistent state.

This patch fixes this by only resetting those fields after the
pskb_may_pull test.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

(cherry picked from commit 17dd759c67f21e34f2156abcf415e1f60605a188)
CVE-2011-2723
BugLink: http://bugs.launchpad.net/bugs/844371
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>