lltable: require the link address length to match the interface

[ojanerif_amd.com]

Summary

lla_rt_output() builds a static LLE from a routing socket message
without checking the sockaddr_dl's sdl_alen. lltable_calc_llheader()
and the if_requestencap() handlers then read if_addrlen bytes from
LLADDR(dl), so a message with a short sdl_alen leaks adjacent bytes of
the (non-zeroed) rtsock buffer into the cached link-layer header, which
for Ethernet is broadcast as the destination MAC of every frame sent to
that entry.

Reject an RTM_ADD whose sdl_alen does not equal the interface address
length, matching the check the netlink neighbor path already performs.
Add a test.

Sponsored by: Advanced Micro Devices, Inc.
Signed-off-by: Osvaldo Janeri Filho <[e-mail]>

Test Plan

tests/sys/net/routing/test_rtsock_lladdr.c gains
rtm_add_v4_lle_bad_alen_failure, which adds an ARP entry over a routing
socket with sdl_alen = 0.

In a vnet jail on a GENERIC-KASAN kernel the unpatched kernel accepts
the message (the EINVAL check fails); with this change the write returns
EINVAL and the test passes. rtm_add_v4_gu_lle_success still passes.