netlink: cope with growing requests

If a request ends up growing beyong the initially allocated space the
netlink functions (such as snl_add_msg_attr_u32()) will allocate a
new buffer. This invalidates the header pointer we can have received
from snl_create_msg_request(). Always use the hdr returned by
snl_finalize_msg().

Reviewed by: melifaro
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42223

(cherry picked from commit 4f8f43b06ed07e96a250855488cc531799d5b78f)