kunit/fortify: Fix mismatched kvalloc()/vfree() usage

The kv*() family of tests were accidentally freeing with vfree() instead
of kvfree(). Use kvfree() instead.

Fixes: 9124a264 ("kunit/fortify: Validate __alloc_size attribute results")
Link: https://lore.kernel.org/r/20240425230619.work.299-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>