netlink: Zero-initialize writer structures allocated on the stack

[markj]

Summary

The prevailing pattern seems to be to simply initialize all fields to
zero. Without this, it's possible to trigger a branch on uninitialized
memory. Specifically, when testing nw->ignore_limit in
nlmsg_refill_buffer().

Initialize the writer structure in a couple of functions where this is
necessary.

Reported by: KMSAN

[melifaro]

Ack, that's a good one!
My initial approach was to perform zero in the -> init() functions (e.g. nlmsg_get_ns_<mbuf|lbuf|buf>), preferably via explicit field initialisation. Apparently the latter one has failed.
ATM I'm leaning towards doing the explicit bzero() in all init functions, but need to think a bit more about it.
To unblock the fix, let's require the explicit writer initialisation for now.