xsk: Diversify return codes in xsk_rcv_check()

Inspired by patch that made xdp_do_redirect() return values for XSKMAP
more meaningful, return -ENXIO instead of -EINVAL for socket being
unbound in xsk_rcv_check() as this is the usual value that is returned
for such event. In turn, it is now possible to easily distinguish what
went wrong, which is a bit harder when for both cases checked, -EINVAL
was returned.

Return codes can be counted in a nice way via bpftrace oneliner that
Jesper has shown:

bpftrace -e 'tracepoint:xdp:xdp_redirect* {@err[-args->err] = count();}'
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/20220413153015.453864-3-maciej.fijalkowski@intel.com

net/xdp/xsk.c

@@ -217,7 +217,7 @@ static bool xsk_is_bound(struct xdp_sock *xs)
 static int xsk_rcv_check(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
 	if (!xsk_is_bound(xs))
-		return -EINVAL;
+		return -ENXIO;
 
 	if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index)
 		return -EINVAL;