nfsd: don't take nfsd4_copy ref for OP_OFFLOAD_STATUS

We're not doing any blocking operations for OP_OFFLOAD_STATUS, so taking
and putting a reference is a waste of effort. Take the client lock,
search for the copy and fetch the wr_bytes_written field and return.

Also, make find_async_copy a static function.

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