notifier: change notifier_from_errno(0) to return NOTIFY_OK

This changes notifier_from_errno(0) to be NOTIFY_OK instead of
NOTIFY_STOP_MASK | NOTIFY_OK.

Currently, the notifiers which return encapsulated errno value have to
do something like this:

	err = do_something(); // returns -errno
	if (err)
		return notifier_from_errno(err);
	else
		return NOTIFY_OK;

This change makes the above code simple:

	err = do_something(); // returns -errno

	return return notifier_from_errno(err);

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/notifier.h

@@ -164,7 +164,10 @@ extern int __srcu_notifier_call_chain(struct srcu_notifier_head *nh,
 /* Encapsulate (negative) errno value (in particular, NOTIFY_BAD <=> EPERM). */
 static inline int notifier_from_errno(int err)
 {
-	return NOTIFY_STOP_MASK | (NOTIFY_OK - err);
+	if (err)
+		return NOTIFY_STOP_MASK | (NOTIFY_OK - err);
+
+	return NOTIFY_OK;
 }
 
 /* Restore (negative) errno value from notify return value. */