sync: Sync .mailmap entries

The kernel repository has a .mailmap file to remap author names and
email addresses to their desired format in Git logs (for details, see
gitmailmap documentation [0]). Alas, this is only visible for author
information when looking at the logs locally, as GitHub does not support
mailmaps at the moment [1].

This commit adds a .mailmap file for libbpf, automatically generated
from the kernel's version. The script to generate the .mailmap is added,
too: it works by grepping email addresses from authors in the
repository, and collecting all lines ending with this address in the
kernel's .mailmap - in other words, all lines where this address is used
as a pattern for a remapping.

To keep the .mailmap up-to-date, add a call to the script to
sync-kernel.sh.

[0] https://git-scm.com/docs/gitmailmap
[1] https://github.com/orgs/community/discussions/22518

Signed-off-by: Quentin Monnet <qmo@kernel.org>

.mailmap

@@ -0,0 +1,16 @@
+Alexei Starovoitov <ast@kernel.org> <alexei.starovoitov@gmail.com>
+Antoine Tenart <atenart@kernel.org> <antoine.tenart@bootlin.com>
+Björn Töpel <bjorn@kernel.org> <bjorn.topel@intel.com>
+Changbin Du <changbin.du@intel.com> <changbin.du@gmail.com>
+Colin Ian King <colin.i.king@gmail.com> <colin.king@canonical.com>
+Dan Carpenter <error27@gmail.com> <dan.carpenter@oracle.com>
+Geliang Tang <geliang@kernel.org> <geliang.tang@suse.com>
+Herbert Xu <herbert@gondor.apana.org.au>
+Jakub Kicinski <kuba@kernel.org> <jakub.kicinski@netronome.com>
+Leo Yan <leo.yan@linux.dev> <leo.yan@linaro.org>
+Mark Starovoytov <mstarovo@pm.me> <mstarovoitov@marvell.com>
+Maxim Mikityanskiy <maxtram95@gmail.com> <maximmi@mellanox.com>
+Maxim Mikityanskiy <maxtram95@gmail.com> <maximmi@nvidia.com>
+Quentin Monnet <qmo@kernel.org> <quentin@isovalent.com>
+Quentin Monnet <qmo@kernel.org> <quentin.monnet@netronome.com>
+Vadim Fedorenko <vadim.fedorenko@linux.dev> <vfedorenko@novek.ru>