net-gro: Fix GRO flush when receiving a GSO packet.

Currently we may merge incorrectly a received GSO packet
or a packet with frag_list into a packet sitting in the
gro_hash list. skb_segment() may crash case because
the assumptions on the skb layout are not met.
The correct behaviour would be to flush the packet in the
gro_hash list and send the received GSO packet directly
afterwards. Commit d61d072e ("net-gro: avoid reorders")
sets NAPI_GRO_CB(skb)->flush in this case, but this is not
checked before merging. This patch makes sure to check this
flag and to not merge in that case.

Fixes: d61d072e ("net-gro: avoid reorders")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>