Revert "SUNRPC: Remove unreachable error condition"

This reverts commit efe57fd.

The assumption that it is impossible to return an ERR pointer from
rpc_run_task() no longer holds due to commit 25cf32a ("SUNRPC:
Handle allocation failure in rpc_new_task()").

Fixes: 25cf32a ('SUNRPC: Handle allocation failure in rpc_new_task()')
Fixes: efe57fd ('SUNRPC: Remove unreachable error condition')
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/clnt.c

@@ -2873,6 +2873,9 @@ int rpc_clnt_test_and_add_xprt(struct rpc_clnt *clnt,
 
 	task = rpc_call_null_helper(clnt, xprt, NULL, RPC_TASK_ASYNC,
 			&rpc_cb_add_xprt_call_ops, data);
+	if (IS_ERR(task))
+		return PTR_ERR(task);
+
 	data->xps->xps_nunique_destaddr_xprts++;
 	rpc_put_task(task);
 success: