nfsd4: recognize callback channel failure earlier

nfsd4: recognize callback channel failure earlier

When the callback channel fails, we inform the client of that by
returning a cb_path_down error the next time it tries to renew its
lease.

If we wait most of a lease period before deciding that a callback has
failed and that the callback channel is down, then we decrease the
chances that the client will find out in time to do anything about it.

So, mark the channel down as soon as we recognize that an rpc has
failed.  However, continue trying to recall delegations anyway, in hopes
it will come back up.  This will prevent more delegations from being
given out, and ensure cb_path_down is returned to renew calls earlier,
while still making the best effort to deliver recalls of existing
delegations.

Also fix a couple comments and remove a dprink that doesn't seem likely
to be useful.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>