net: Introduce SO_INCOMING_NAPI_ID

This socket option returns the NAPI ID associated with the queue on which
the last frame is received. This information can be used by the apps to
split the incoming flows among the threads based on the Rx queue on which
they are received.

If the NAPI ID actually represents a sender_cpu then the value is ignored
and 0 is returned.

Signed-off-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>