sk_buff: introduce 'slow_gro' flags

The new flag tracks if any state field is set, so that
GRO requires 'unusual'/slow prepare steps.

Set such flag when a ct entry is attached to the skb,
and never clear it.

The new bit uses an existing hole into the sk_buff struct

RFC -> v1:
 - use a single state bit, never clear it
 - avoid moving the _nfct field

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>