kunit: Allow passing function pointer to kunit_activate_static_stub()

Swap the arguments to typecheck_fn() in kunit_activate_static_stub()
so that real_fn_addr can be either the function itself or a pointer
to that function.

This is useful to simplify redirecting static functions in a module.
Having to pass the actual function meant that it must be exported
from the module. Either making the 'static' and EXPORT_SYMBOL*()
conditional (which makes the code messy), or change it to always
exported (which increases the export namespace and prevents the
compiler inlining a trivial stub function in non-test builds).

With the original definition of kunit_activate_static_stub() the
address of real_fn_addr was passed to typecheck_fn() as the type to
be passed. This meant that if real_fn_addr was a pointer-to-function
it would resolve to a ** instead of a *, giving an error like this:

   error: initialization of ‘int (**)(int)’ from incompatible pointer
   type ‘int (*)(int)’ [-Werror=incompatible-pointer-types]
   kunit_activate_static_stub(test, add_one_fn_ptr, subtract_one);
      |                             ^~~~~~~~~~~~
   ./include/linux/typecheck.h:21:25: note: in definition of macro
   ‘typecheck_fn’
   21 | ({ typeof(type) __tmp = function; \

Swapping the arguments to typecheck_fn makes it take the type of a
pointer to the replacement function. Either a function or a pointer
to function can be assigned to that. For example:

static int some_function(int x)
{
    /* whatever */
}

int (* some_function_ptr)(int) = some_function;

static int replacement(int x)
{
    /* whatever */
}

Then:
  kunit_activate_static_stub(test, some_function, replacement);
yields:
  typecheck_fn(typeof(&replacement), some_function);

and:
  kunit_activate_static_stub(test, some_function_ptr, replacement);
yields:
  typecheck_fn(typeof(&replacement), some_function_ptr);

The two typecheck_fn() then resolve to:

  int (*__tmp)(int) = some_function;
and
  int (*__tmp)(int) = some_function_ptr;

Both of these are valid. In the first case the compiler inserts
an implicit '&' to take the address of the supplied function, and
in the second case the RHS is already a pointer to the same type.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

include/kunit/static_stub.h

@@ -93,7 +93,7 @@ void __kunit_activate_static_stub(struct kunit *test,
  * The redirection can be disabled again with kunit_deactivate_static_stub().
  */
 #define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do {	\
-	typecheck_fn(typeof(&real_fn_addr), replacement_addr);			\
+	typecheck_fn(typeof(&replacement_addr), real_fn_addr);			\
 	__kunit_activate_static_stub(test, real_fn_addr, replacement_addr);	\
 } while (0)