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>

drivers/net/wan/cosa.c

@@ -993,8 +993,8 @@ static struct fasync_struct *fasync[256] = { NULL, };
 static int cosa_fasync(struct inode *inode, struct file *file, int on)
 {
         int port = iminor(inode);
-        int rv = fasync_helper(inode, file, on, &fasync[port]);
-        return rv < 0 ? rv : 0;
+
+	return fasync_helper(inode, file, on, &fasync[port]);
 }
 #endif