nfsd: update callback done processing

Instead of having the convention where individual nfsd4_callback_ops->done
operations return -1 to indicate the callback path is down, move the check
to nfsd4_cb_done.  Only mark the callback path down on transport-level
errors, not NFS-level errors.

The existing logic causes the server to set SEQ4_STATUS_CB_PATH_DOWN
just because the client returned an error to a CB_RECALL for a
delegation that the client had already done a FREE_STATEID for.  But
clearly that error doesn't mean that there's anything wrong with the
backchannel.

Additionally, handle NFS4ERR_DELAY in nfsd4_cb_recall_done.  The client
returns NFS4ERR_DELAY if it is already in the process of returning the
delegation.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>