gro: Enter slow-path if there is no tailroom

commit 1272ce87fa017ca4cf32920764d879656b7a005a upstream.

The GRO path has a fast-path where we avoid calling pskb_may_pull
and pskb_expand by directly accessing frag0.  However, this should
only be done if we have enough tailroom in the skb as otherwise
we'll have to expand it later anyway.

This patch adds the check by capping frag0_len with the skb tailroom.

Fixes: cb18978c ("gro: Open-code final pskb_may_pull")
Reported-by: Slava Shwartsman <slavash@mellanox.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>