NFSD: Initialize struct nfsd4_copy earlier

Ensure the refcount and async_copies fields are initialized early.
cleanup_async_copy() will reference these fields if an error occurs
in nfsd4_copy(). If they are not correctly initialized, at the very
least, a refcount underflow occurs.

Reported-by: Olga Kornievskaia <okorniev@redhat.com>
Fixes: aadc3bbe ("NFSD: Limit the number of concurrent async COPY operations")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>