[PATCH v2] linux: Enforce zero fill of siginfo_t

[Florian Weimer]

* Adhemerval Zanella:

> The glibc siginfo struct is larger than the kABI one, and the kernel
> enforces that for an unknown si_code, all the extra bytes should be 0
> (post_copy_siginfo_from_user).

I still think the explanation of this change is wrong (I think it's that
designated initializers with unions do not necessary zero-initialize
inactive members), but the code change itself is correct.

Thanks,
Florian