sunrpc: remove unnecessary test in rpc_task_set_client()

In rpc_task_set_client(), testing for a NULL clnt is not necessary, as
clnt should always be a valid pointer to a rpc_client.

Signed-off-by: Thiago Rafael Becker <trbecker@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>