Documentation: xsk: correct the obsolete references and examples

The modified lines are mainly related to the following commits[1][2]
which remove those tests and examples. Since samples/bpf has been
deprecated, we can refer to more examples that are easily searched
in the various xdp-projects, like the following link:
https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example

[1]
commit f366006342 ("libbpf: move xsk.{c,h} into selftests/bpf")
[2]
commit cfb5a2dbf1 ("bpf, samples: Remove AF_XDP samples")

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250708062907.11557-1-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/af_xdp.rst

@@ -209,13 +209,10 @@ Libbpf
 
 Libbpf is a helper library for eBPF and XDP that makes using these
 technologies a lot simpler. It also contains specific helper functions
-in tools/lib/bpf/xsk.h for facilitating the use of AF_XDP. It
-contains two types of functions: those that can be used to make the
-setup of AF_XDP socket easier and ones that can be used in the data
-plane to access the rings safely and quickly. To see an example on how
-to use this API, please take a look at the sample application in
-samples/bpf/xdpsock_usr.c which uses libbpf for both setup and data
-plane operations.
+in tools/testing/selftests/bpf/xsk.h for facilitating the use of
+AF_XDP. It contains two types of functions: those that can be used to
+make the setup of AF_XDP socket easier and ones that can be used in the
+data plane to access the rings safely and quickly.
 
 We recommend that you use this library unless you have become a power
 user. It will make your program a lot simpler.
@@ -372,9 +369,8 @@ needs to explicitly notify the kernel to send any packets put on the
 TX ring. This can be accomplished either by a poll() call, as in the
 RX path, or by calling sendto().
 
-An example of how to use this flag can be found in
-samples/bpf/xdpsock_user.c. An example with the use of libbpf helpers
-would look like this for the TX path:
+An example with the use of libbpf helpers would look like this for the
+TX path:
 
 .. code-block:: c
 
@@ -549,12 +545,12 @@ later in this document.
 Usage
 -----
 
-In order to use AF_XDP sockets two parts are needed. The
-user-space application and the XDP program. For a complete setup and
-usage example, please refer to the sample application. The user-space
-side is xdpsock_user.c and the XDP side is part of libbpf.
+In order to use AF_XDP sockets two parts are needed. The user-space
+application and the XDP program. For a complete setup and usage example,
+please refer to the xdp-project at
+https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example.
 
-The XDP code sample included in tools/lib/bpf/xsk.c is the following:
+The XDP code sample is the following:
 
 .. code-block:: c
 
@@ -752,11 +748,12 @@ to facilitate extending a zero-copy driver with multi-buffer support.
 
 Sample application
 ==================
-
-There is a xdpsock benchmarking/test application included that
-demonstrates how to use AF_XDP sockets with private UMEMs. Say that
-you would like your UDP traffic from port 4242 to end up in queue 16,
-that we will enable AF_XDP on. Here, we use ethtool for this::
+There is a xdpsock benchmarking/test application that can be found at
+https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example
+that demonstrates how to use AF_XDP sockets with private
+UMEMs. Say that you would like your UDP traffic from port 4242 to end
+up in queue 16, that we will enable AF_XDP on. Here, we use ethtool
+for this::
 
       ethtool -N p3p2 rx-flow-hash udp4 fn
       ethtool -N p3p2 flow-type udp4 src-port 4242 dst-port 4242 \
@@ -773,7 +770,7 @@ can be displayed with "-h", as usual.
 This sample application uses libbpf to make the setup and usage of
 AF_XDP simpler. If you want to know how the raw uapi of AF_XDP is
 really used to make something more advanced, take a look at the libbpf
-code in tools/lib/bpf/xsk.[ch].
+code in tools/testing/selftests/bpf/xsk.[ch].
 
 FAQ
 =======