scripts: kernel-doc: Always increment warnings counter

Some warnings do not increment the warnings counter making the behavior
of running kernel-doc with -Werror unlogical as some warnings will be
generated but not treated as errors.

Fix this by creating a helper function that always incrementing the
warnings counter every time a warning is emitted. There is one location
in get_sphinx_version() where a warning is not touched as it concerns
the execution environment of the kernel-doc and not the documentation
being processed.

Incrementing the counter only have effect when running kernel-doc in
either verbose mode (-v or environment variable KBUILD_VERBOSE) or when
treating warnings as errors (-Werror or environment variable
KDOC_WERROR). In both cases the number of warnings printed is printed to
stderr and for the later the exit code of kernel-doc is non-zero if
warnings where encountered.

Simple test case to demo one of the warnings,

    $ cat test.c
    /**
     * foo() - Description
     */
    int bar();

    # Without this change
    $ ./scripts/kernel-doc -Werror -none test.c
    test.c:4: warning: expecting prototype for foo(). Prototype was for
    bar() instead

    # With this change
    $ ./scripts/kernel-doc -Werror -none test.c
    test.c:4: warning: expecting prototype for foo(). Prototype was for
    bar() instead
    1 warnings as Errors

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Link: https://lore.kernel.org/r/20220613090510.3088294-1-niklas.soderlund@corigine.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>