netlink: Zero-initialize writer structures allocated on the stack

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
Reviewed by: melifaro
Approved by: re(cperciva)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38213

(cherry picked from commit 7a78ae88659e0f6a901574d17672d1ccdc3e971b)
(cherry picked from commit 2de541757e0a2e25ce974b3c4c29bc8b753b8b6a)