NFSD add nfs4 inter ssc to nfsd4_copy

Given a universal address, mount the source server from the destination
server.  Use an internal mount. Call the NFS client nfs42_ssc_open to
obtain the NFS struct file suitable for nfsd_copy_range.

Ability to do "inter" server-to-server depends on the an nfsd kernel
parameter "inter_copy_offload_enable".

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>

fs/nfsd/xdr4.h

@@ -549,7 +549,12 @@ struct nfsd4_copy {
 	struct task_struct	*copy_task;
 	refcount_t		refcount;
 	bool			stopped;
+
+	struct vfsmount		*ss_mnt;
+	struct nfs_fh		c_fh;
+	nfs4_stateid		stateid;
 };
+extern bool inter_copy_offload_enable;
 
 struct nfsd4_seek {
 	/* request */