netlink: Fix indentation in netlink_message_writer.c

This file is indented with a mixture of tabs and spaces. No functional
change intended.

Reviewed by: melifaro
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38100

(cherry picked from commit 35472cb60ab95b4d99981492a87443022181eac2)