When the receive buffer is too small, we easily can hit ENOBUFS during recvmsg()
and need to resync the platform cache. But even worse, we possibly also loose
ACKs for pending netlink requests so that requests seem to fail (although they
might have succeeded).
Avoid that harder by increasing the buffer size to 8MB. This is also
done by networkd:
be660c371b
f00c472ee8