6.2 mtu now limits size of incomming packet

[Mike Karels]

> In -CURRENT my changes to the ethernet input path maintain the use of 
> ETHER_MAX_FRAME() however the check is folded under #ifdef DIAGNOSTIC. I 
> don't recall adding this conditional or touching it so it seems to be 
> something which was already thereo radded by someone else.

It has been there at least since 6.0.  The issue is that ETHER_MAX_FRAME
is computed using ifp->if_mtu, as opposed to something like ETHER_MAX_LEN.
This is under #ifdef DIAGNOSTIC in -current, not in -stable.

> Could be pilot error; its use in -CURRENT seems to apply strictly to the 
> use of large-receive offload (LRO).

LRO relaxes the requirement, otherwise LRO packets would never pass the
check.

		Mike