NFSD: Block DESTROY_CLIENTID only when there are ongoing async COPY operations

Currently __destroy_client() consults the nfs4_client's async_copies
list to determine whether there are ongoing async COPY operations.
However, NFSD now keeps copy state in that list even when the
async copy has completed, to enable OFFLOAD_STATUS to find the
COPY results for a while after the COPY has completed.

DESTROY_CLIENTID should not be blocked if the client's async_copies
list contains state for only completed copy operations.

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