nfs42: do not fail with EIO if ssc returns NFS4ERR_OFFLOAD_DENIED

The NFSv4.2 server even if supports intra-SSC might prefer that for
a particular file a classic copy is performed. As returning ENOTSUPP
will clear the SSC capability of the server by the client, server
might return NFS4ERR_OFFLOAD_DENIED (well, spec talks about remote
servers there).

Update nfs42_proc_copy to handle NFS4ERR_OFFLOAD_DENIED as ENOTSUPP,
but without clearing NFS_CAP_COPY bit.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>