net: Properly unlink GRO packets on overflow.

[ Upstream commti ece23711dd956cd5053c9cb03e9fe0668f9c8894 ]

Just like with normal GRO processing, we have to initialize
skb->next to NULL when we unlink overflow packets from the
GRO hash lists.

Fixes: d4546c25 ("net: Convert GRO SKB handling to list_head.")
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>