esp: Add a software GRO codepath

This patch adds GRO callbacks for ESP on ipv4 and ipv6.

In case the GRO layer detects an ESP packet, the
esp4_gro_receive() function calls the xfrm input layer
which decapsulates the packet and reinject it into
layer 2 by calling netif_rx(). We use on bit of the
sk_buff to flag xfrm_gro. If this bit is set, the
process_backlog() function calls napi_gro_receive()
intead of __netif_receive_skb(). We could avoid the
usage of xfrm_gro if we could call napi_gro_receive()
unconditionaly from process_backlog().

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>