net: add socket option for low latency polling

adds a socket option for low latency polling.
This allows overriding the global sysctl value with a per-socket one.
Unexport sysctl_net_ll_poll since for now it's not needed in modules.

Signed-off-by: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -230,6 +230,7 @@ struct cg_proto;
   *	@sk_wmem_queued: persistent queue size
   *	@sk_forward_alloc: space allocated forward
   *	@sk_napi_id: id of the last napi context to receive data for sk
+  *	@sk_ll_usec: usecs to busypoll when there is no data
   *	@sk_allocation: allocation mode
   *	@sk_sndbuf: size of send buffer in bytes
   *	@sk_flags: %SO_LINGER (l_onoff), %SO_BROADCAST, %SO_KEEPALIVE,
@@ -328,6 +329,7 @@ struct sock {
 #endif
 #ifdef CONFIG_NET_LL_RX_POLL
 	unsigned int		sk_napi_id;
+	unsigned int		sk_ll_usec;
 #endif
 	atomic_t		sk_drops;
 	int			sk_rcvbuf;