ospf6d: fix rx/tx buffer sizes

ospf6d: fix rx/tx buffer sizes

OSPFv3 packets can be fragmented and up to 64k long, regardless of
interface MTU.  Trying to size these buffers to MTU is just plain wrong.
To not make this a super intrusive change during the 8.3 release freeze,
just code this into ospf6_iobuf_size().

Since the buffer is now always 64k, don't waste time zeroing the entire
thing in receive;  instead just zero kind of a "sled" of 128 bytes after
the buffer as a security precaution.

Fixes: #11298
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>