net: Add a skb_gro_flush_final helper.

Add a skb_gro_flush_final helper to prepare for  consuming
skbs in call_gro_receive. We will extend this helper to not
touch the skb if the skb is consumed by a gro callback with
a followup patch. We need this to handle the upcomming IPsec
ESP callbacks as they reinject the skb to the napi_gro_receive
asynchronous. The handler is used in all gro_receive functions
that can call the ESP gro handlers.

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