docs: networking: convert filter.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- use footnote markup;
- mark tables as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/bpf/bpf_asm.c

@@ -11,7 +11,7 @@
  *
  * How to get into it:
  *
- * 1) read Documentation/networking/filter.txt
+ * 1) read Documentation/networking/filter.rst
  * 2) Run `bpf_asm [-c] <filter-prog file>` to translate into binary
  *    blob that is loadable with xt_bpf, cls_bpf et al. Note: -c will
  *    pretty print a C-like construct.

tools/bpf/bpf_dbg.c

@@ -13,7 +13,7 @@
  * for making a verdict when multiple simple BPF programs are combined
  * into one in order to prevent parsing same headers multiple times.
  *
- * More on how to debug BPF opcodes see Documentation/networking/filter.txt
+ * More on how to debug BPF opcodes see Documentation/networking/filter.rst
  * which is the main document on BPF. Mini howto for getting started:
  *
  *  1) `./bpf_dbg` to enter the shell (shell cmds denoted with '>'):