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Make the nfs_callback_up()/down() prototypes just do nothing if NFS4 is not
enabled.  Also make the down function void type since we can't really do
anything if it fails.
Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>