NFSD: fix missing refcount in nfsd4_copy by nfsd4_do_async_copy

Need to initialize nfsd4_copy's refcount to 1 to avoid use-after-free
warning when nfs4_put_copy is called from nfsd4_cb_offload_release.

Fixes: ce0887ac ("NFSD add nfs4 inter ssc to nfsd4_copy")
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>