netlink: cope with growing requests

[kp]

Summary

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().

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")