Documentation/bpf: Use valid and new links in index.rst

There exists an error "404 Not Found" when I click the html link of
"Documentation/networking/filter.rst" in the BPF documentation [1],
fix it.

Additionally, use the new links about "BPF and XDP Reference Guide"
and "bpf(2)" to avoid redirects.

[1] https://www.kernel.org/doc/html/latest/bpf/

Fixes: d9b9170a26 ("docs: bpf: Rename README.rst to index.rst")
Fixes: cb3f0d56e1 ("docs: networking: convert filter.txt to ReST")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1596184142-18476-1-git-send-email-yangtiezhu@loongson.cn

Documentation/networking/filter.rst

@@ -1,5 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0
 
+.. _networking-filter:
+
 =======================================================
 Linux Socket Filtering aka Berkeley Packet Filter (BPF)
 =======================================================