NFS deadlock on 9.2-Beta1

[J David]

If it is helpful, we have 25 nodes testing the 9.2-BETA1 build and
without especially trying to exercise this bug, we found
sendfile()-using processes deadlocked in WCHAN newnfs on 5 of the 25
nodes.  The ones with highest uptime (about 3 days) seem most
affected, so it does seem like a "sooner or later" type of thing.
Hopefully the fix is easy and it won't be an issue, but it definitely
does seem like a problem 9.2-RELEASE would be better off without.

Unfortunately we are not in a position to capture the requested
debugging information at this time; none of those nodes are running a
debug version of the kernel.  If Michael is unable to get the
information as he hopes, we can try to do that, possibly over the
weekend.  For the time being, we will convert half the machines to
rollback r250907 to try to confirm that resolves the issue.

Thanks all!  If one has to encounter a problem like this, it is nice
to come to the list and find the research already so well underway!