xsk: add Tx queue setup and mmap support

Another setsockopt (XDP_TX_QUEUE) is added to let the process allocate
a queue, where the user process can pass frames to be transmitted by
the kernel.

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

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/xdp_sock.h

@@ -30,6 +30,7 @@ struct xdp_sock {
 	struct xdp_umem *umem;
 	struct list_head flush_node;
 	u16 queue_id;
+	struct xsk_queue *tx ____cacheline_aligned_in_smp;
 	/* Protects multiple processes in the control path */
 	struct mutex mutex;
 	u64 rx_dropped;