xsk: Initialise xskb free_list_node

This commit initialises the xskb's free_list_node when the xskb is
allocated. This prevents a potential false negative returned from a call
to list_empty for that node, such as the one introduced in commit
199d983b ("xsk: Fix crash on double free in buffer pool")

In my environment this issue caused packets to not be received by
the xdpsock application if the traffic was running prior to application
launch. This happened when the first batch of packets failed the xskmap
lookup and XDP_PASS was returned from the bpf program. This action is
handled in the i40e zc driver (and others) by allocating an skbuff,
freeing the xdp_buff and adding the associated xskb to the
xsk_buff_pool's free_list if it hadn't been added already. Without this
fix, the xskb is not added to the free_list because the check to determine
if it was added already returns an invalid positive result. Later, this
caused allocation errors in the driver and the failure to receive packets.

Fixes: 199d983b ("xsk: Fix crash on double free in buffer pool")
Fixes: 2b43470a ("xsk: Introduce AF_XDP buffer allocation API")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20211220155250.2746-1-ciara.loftus@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/xdp/xsk_buff_pool.c

@@ -83,6 +83,7 @@ struct xsk_buff_pool *xp_create_and_assign_umem(struct xdp_sock *xs,
 		xskb = &pool->heads[i];
 		xskb->pool = pool;
 		xskb->xdp.frame_sz = umem->chunk_size - umem->headroom;
+		INIT_LIST_HEAD(&xskb->free_list_node);
 		if (pool->unaligned)
 			pool->free_heads[i] = xskb;
 		else