NFS: Add RPC callouts to start NFSv4.0 synchronous requests

Refactor nfs4_call_sync_sequence() so it is used for NFSv4.0 now.
The RPC callouts will house transport blocking logic similar to
NFSv4.1 sessions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index 0feb238..bf2184e 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -49,6 +49,7 @@
 			struct nfs_fsinfo *);
 	int	(*free_lock_state)(struct nfs_server *,
 			struct nfs4_lock_state *);
+	const struct rpc_call_ops *call_sync_ops;
 	const struct nfs4_state_recovery_ops *reboot_recovery_ops;
 	const struct nfs4_state_recovery_ops *nograce_recovery_ops;
 	const struct nfs4_state_maintenance_ops *state_renewal_ops;