xennet0: rx no buf after upgrading to NetBSD 7

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've upgraded a web-server from NetBSD 5/i386 to 7/amd64 and now I'm 
frequently getting "xennet0: rx no buf". I don't think the server is any 
busier than it was before, but I guess there could be sporadic DoS 
attacks on it.

Of course, when I look now, it's all idle:

# netstat -m
5 mbufs in use:
        1 mbufs allocated to data
        3 mbufs allocated to packet headers
        1 mbufs allocated to socket names and addresses
93318 calls to protocol drain routines

kern.mbuf.nmbclusters = 98304

I looked at this thread:
https://mail-index.netbsd.org/port-xen/2015/03/28/msg008611.html

While it may explain what is behind the message, it's short on ways to 
mitigate it.

--
Stephen