randstruct: Check member structs in is_pure_ops_struct()

While no uses in the kernel triggered this case, it was possible to have
a false negative where a struct contains other structs which contain only
function pointers because of unreachable code in is_pure_ops_struct().

Signed-off-by: Joonwon Kang <kjw1627@gmail.com>
Link: https://lore.kernel.org/r/20190727155841.GA13586@host


Fixes: 313dd1b6 ("gcc-plugins: Add the randstruct plugin")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>