Although there is no explicit coding style for the BPF code, several
developers tend to follow the kernel coding style conventions [0]. To
try and maintain a consistent style all over the code, this commit adds
kernel's checkpatch.pl to the list of checks performed on the source as
part as BPF's "make check".

Some details on the check:

- We only run checkpatch.pl on the bpf/ directory.

- A new "checkpatch" BPF make target is created, and added as a
  dependency to "check".

- However, we only run it on the latest changes by default, meaning the
  changes since newest Git parent reference plus the changes returned by
  "git diff HEAD".

- For manual checks, the bash script used to launch checkpatch has an
  option to run the checks on all the bpf/ source code, independently of
  the Git history.

- GitHub action runs checkpatch.pl on all commits from the PR. There is
  no mechanism to bypass checkpatch errors, Janitors will have to decide
  if issues reported by checkpatch should be ignored.

The checkpatch.pl and spelling.txt files (both under GPLv2) are directly
copied from the kernel repository, as per commit f436a58e2619
("checkpatch: fix CONST_STRUCT when const_structs.checkpatch is
missing") (in linux-next as of the creation of this patch). File
deprecated_terms.txt is created from this PR, and populated in prevision
of the changes proposed in cilium#12206. The script checkpatch.sh (bash
script) launches checkpatch.pl with the relevant arguments and options.
COPYING file is copied from bpf/COPYING, to make sure people get a
version of the license boilerplate with the checkpatch code.

[0] https://www.kernel.org/doc/html/v5.6/process/coding-style.html

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Although there is no explicit coding style for the BPF code, several
developers tend to follow the kernel coding style conventions [0]. To
try and maintain a consistent style all over the code, this commit adds
kernel's checkpatch.pl to the list of checks performed on the source as
part as BPF's "make check".

Some details on the check:

- We only run checkpatch.pl on the bpf/ directory.

- A new "checkpatch" BPF make target is created, and added as a
  dependency to "check".

- However, we only run it on the latest changes by default, meaning the
  changes since newest Git parent reference plus the changes returned by
  "git diff HEAD".

- For manual checks, the bash script used to launch checkpatch has an
  option to run the checks on all the bpf/ source code, independently of
  the Git history.

- GitHub action runs checkpatch.pl on all commits from the PR. There is
  no mechanism to bypass checkpatch errors, Janitors will have to decide
  if issues reported by checkpatch should be ignored.

The checkpatch.pl and spelling.txt files (both under GPLv2) are directly
copied from the kernel repository, as per commit f436a58e2619
("checkpatch: fix CONST_STRUCT when const_structs.checkpatch is
missing") (in linux-next as of the creation of this patch). File
deprecated_terms.txt is created from this PR, and populated in prevision
of the changes proposed in #12206. The script checkpatch.sh (bash
script) launches checkpatch.pl with the relevant arguments and options.
COPYING file is copied from bpf/COPYING, to make sure people get a
version of the license boilerplate with the checkpatch code.

[0] https://www.kernel.org/doc/html/v5.6/process/coding-style.html

Signed-off-by: Quentin Monnet <quentin@isovalent.com>