docs: maintainers_include: fix support for O=dir

docs: maintainers_include: fix support for O=dir

os.path.relpath() will do the wrong thing with O=dir, as the build
system uses "cd <dir>" internally.

Solve it by using app.srcdir, which, on normal cases, point to
Documentation/, or, when SPHINXDIRS=process, it will be set with
Documentation/process.

While here, remove a dead code while writing maintainer profiles,
as now all entries should have both profile and entry.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/linux-doc/88335220-3527-4b1f-9500-417f7ebb7a02@infradead.org/T/#m6854cbd8d30e2c5d3e8c4173bae1c3d6922ff970
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <193792d73d08d9f8b9faa8b3df8a1a7439344a28.1777295258.git.mchehab+huawei@kernel.org>