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>