em driver - issue #2

[EM1897@xxxxxxx]

Ok, now thats its not crashing anymore, on to the next problem. After
starting the traffic generator, I now get the expected 

"All mbuf clusters exhausted, please see tuning(7)"

message. However it apparently locks up the controller, or at least
it doesn't recover as expected. Once I stop the traffic generator,
other interfaces on the dragonfly machine operate normally, but
the one that was being pounded cannot receive. If I initiate a 
ping from the dfly box out of the problem controller, then it starts
working ok. I also get a

"Limiting ICMP unreach response from 11768 to 200 pps"

which implies that quite a few packets get "stuck" somewhere and
are all released at once when a transmit is initiated, since
the traffic had stopped arriving long before.

I'll have a chance tomorrow perhaps to dig a bit deeper, but any ideas
about what might have been changed to cause this  (as this isnt a 
problem in freebsd) would be helpful. I can also try it on an fxp
interface to see if its an em problem or a system-wide problem.