nfsd4: shut down callback queue outside state lock

nfsd4: shut down callback queue outside state lock

This reportedly causes a lockdep warning on nfsd shutdown.  That looks
like a false positive to me, but there's no reason why this needs the
state lock anyway.

Reported-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>