9p: make cryptic unknown error from server less scary

Right now when we get an error string from the server that we can't
map we report a cryptic error that actually makes it look like we are
reporting a problem with the client.  This changes the text of the log
message to clarify where the error is coming from.
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

net/9p/error.c

@@ -237,8 +237,8 @@ int p9_errstr2errno(char *errstr, int len)
 	if (errno == 0) {
 		/* TODO: if error isn't found, add it dynamically */
 		errstr[len] = 0;
-		printk(KERN_ERR "%s: errstr :%s: not found\n", __func__,
-		       errstr);
+		printk(KERN_ERR "%s: server reported unknown error %s\n",
+			__func__, errstr);
 		errno = 1;
 	}