process of being unmounted. This allows forced NFS unmounts to
complete even if there are processes stuck holding the mnt_lock
while the server is down. The mechanism is not ideal in that there
is a small chance we might accidentally cancel requests during a
failed non-forced unmount attempt on that filesystem, but this
is not really a big problem.
Also, move the tsleep() in nfs_nmcancelreqs() so that we do not
sleep in the case where there are no requests to be cancelled.