Some drivers use copybreak to copy tiny frames into smaller skb,
and this smaller skb might not have skb->head_frag set for various
reasons.

skb_gro_receive() currently doesn't allow to aggregate the smaller skb
into the previous GRO packet if this GRO packet has at least 2 MSS in
it.

Following workload easily demonstrates the problem.

netperf -t TCP_RR -H target -- -r 3000,3000

(tcpdump shows one GRO packet with 2 MSS, plus one additional packet of
104 bytes that should have been appended.)

It turns out that we can remove code from skb_gro_receive(), because
commit 8a29111c ("net: gro: allow to build full sized skb") and its
followups removed the assumption that a GRO packet with a frag_list had
to have an empty head.

Removing this code allows the aggregation of the last (incomplete) frame
in some RPC workloads. Note that tcp_gro_receive() already takes care of
forcing a flush if necessary, including this case.

If we want to avoid using frag_list in the first place (in forwarding
workloads for example, as the outgoing NIC is generally not able to cope
with skbs having a frag_list), we need to address this separately.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>