svcrpc: implement O_NONBLOCK behavior for use-gss-proxy

Somebody noticed LTP was complaining about O_NONBLOCK opens of
/proc/net/rpc/use-gss-proxy succeeding and then a following read
hanging.

I'm not convinced LTP really has any business opening random proc files
and expecting them to behave a certain way.  Maybe this isn't really a
bug.

But in any case the O_NONBLOCK behavior could be useful for someone that
wants to test whether gss-proxy is up without waiting.
Reported-by: NJan Stancek <jstancek@redhat.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1317,10 +1317,12 @@ static inline bool gssp_ready(struct sunrpc_net *sn)
 	return false;
 }
 
-static int wait_for_gss_proxy(struct net *net)
+static int wait_for_gss_proxy(struct net *net, struct file *file)
 {
 	struct sunrpc_net *sn = net_generic(net, sunrpc_net_id);
 
+	if (file->f_flags & O_NONBLOCK && !gssp_ready(sn))
+		return -EAGAIN;
 	return wait_event_interruptible(sn->gssp_wq, gssp_ready(sn));
 }
 
@@ -1362,7 +1364,7 @@ static ssize_t read_gssp(struct file *file, char __user *buf,
 	size_t len;
 	int ret;
 
-	ret = wait_for_gss_proxy(net);
+	ret = wait_for_gss_proxy(net, file);
 	if (ret)
 		return ret;