kunit: Expose 'static stub' API to redirect functions

Add a simple way of redirecting calls to functions by including a
special prologue in the "real" function which checks to see if the
replacement function should be called (and, if so, calls it).

To redirect calls to a function, make the first (non-declaration) line
of the function:

KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);

(This will compile away to nothing if KUnit is not enabled, otherwise it
will check if a redirection is active, call the replacement function,
and return. This check is protected by a static branch, so has very
little overhead when there are no KUnit tests running.)

Calls to the real function can be redirected to a replacement using:

kunit_activate_static_stub(test, real_fn, replacement_fn);

The redirection will only affect calls made from within the kthread of
the current test, and will be automatically disabled when the test
completes. It can also be manually disabled with
kunit_deactivate_static_stub().

The 'example' KUnit test suite has a more complete example.

Co-developed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>