xsk: add Rx queue setup and mmap support

Another setsockopt (XDP_RX_QUEUE) is added to let the process allocate
a queue, where the kernel can pass completed Rx frames from the kernel
to user process.

The mmapping of the queue is done using the XDP_PGOFF_RX_QUEUE offset.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index 94785f5..db9a321d 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -18,11 +18,15 @@
 #include <linux/mutex.h>
 #include <net/sock.h>
 
+struct net_device;
+struct xsk_queue;
 struct xdp_umem;
 
 struct xdp_sock {
 	/* struct sock must be the first member of struct xdp_sock */
 	struct sock sk;
+	struct xsk_queue *rx;
+	struct net_device *dev;
 	struct xdp_umem *umem;
 	/* Protects multiple processes in the control path */
 	struct mutex mutex;