scripts/bpf: Set date attribute for bpf-helpers(7) man page

The bpf-helpers(7) manual page shipped in the man-pages project is
generated from the documentation contained in the BPF UAPI header, in
the Linux repository, parsed by script/bpf_doc.py and then fed to
rst2man.

The man page should contain the date of last modification of the
documentation. This commit adds the relevant date when generating the
page.

Before:

    $ ./scripts/bpf_doc.py helpers | rst2man | grep '\.TH'
    .TH BPF-HELPERS 7 "" "Linux v5.19-14022-g30d2a4d74e11" ""

After:

    $ ./scripts/bpf_doc.py helpers | rst2man | grep '\.TH'
    .TH BPF-HELPERS 7 "2022-08-15" "Linux v5.19-14022-g30d2a4d74e11" ""

We get the version by using "git log" to look for the commit date of the
latest change to the section of the BPF header containing the
documentation. If the command fails, we just skip the date field. and
keep generating the page.

Reported-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alejandro Colomar <alx.manpages@gmail.com>
Link: https://lore.kernel.org/bpf/20220823155327.98888-2-quentin@isovalent.com