Rationalize fasync return values

Most fasync implementations do something like:

     return fasync_helper(...);

But fasync_helper() will return a positive value at times - a feature used
in at least one place.  Thus, a number of other drivers do:

     err = fasync_helper(...);
     if (err < 0)
             return err;
     return 0;

In the interests of consistency and more concise code, it makes sense to
map positive return values onto zero where ->fasync() is called.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

fs/ioctl.c

@@ -432,7 +432,7 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
 		else
 			error = -ENOTTY;
 	}
-	return error;
+	return error < 0 ? error : 0;
 }
 
 static int ioctl_fsfreeze(struct file *filp)