Revert "NFSD: Force all NFSv4.2 COPY requests to be synchronous"

Revert "NFSD: Force all NFSv4.2 COPY requests to be synchronous"

In the past several kernel releases, we've made NFSv4.2 async copy
reliable:
 - The Linux NFS client and server now both implement and use the
   NFSv4.2 OFFLOAD_STATUS operation
 - The Linux NFS server keeps copy stateids around longer
 - The Linux NFS client and server now both implement referring call
   lists

And resilient against DoS:
 - The Linux NFS server limits the number of concurrent async copy
   operations

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>