lib/vsprintf.c: fix vscnprintf() if @size is == 0

vscnprintf() should return 0 if @size is == 0.  Update the comment for it,
as @size is unsigned.

This change based on the code of commit
b903c0b8899b46829a9b80ba55b61079b35940ec ("lib: fix scnprintf() if @size
is == 0") moves the real fix into vscnprinf() from scnprintf() and makes
scnprintf() call vscnprintf(), thus avoid code duplication.

Signed-off-by: Anton Arapov <aarapov@redhat.com>
Acked-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/vsprintf.c

@@ -1473,7 +1473,7 @@ EXPORT_SYMBOL(vsnprintf);
  * @args: Arguments for the format string
  *
  * The return value is the number of characters which have been written into
- * the @buf not including the trailing '\0'. If @size is <= 0 the function
+ * the @buf not including the trailing '\0'. If @size is == 0 the function
  * returns 0.
  *
  * Call this function if you are already dealing with a va_list.
@@ -1487,7 +1487,11 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
 
 	i = vsnprintf(buf, size, fmt, args);
 
-	return (i >= size) ? (size - 1) : i;
+	if (likely(i < size))
+		return i;
+	if (size != 0)
+		return size - 1;
+	return 0;
 }
 EXPORT_SYMBOL(vscnprintf);
 
@@ -1535,14 +1539,10 @@ int scnprintf(char *buf, size_t size, const char *fmt, ...)
 	int i;
 
 	va_start(args, fmt);
-	i = vsnprintf(buf, size, fmt, args);
+	i = vscnprintf(buf, size, fmt, args);
 	va_end(args);
 
-	if (likely(i < size))
-		return i;
-	if (size != 0)
-		return size - 1;
-	return 0;
+	return i;
 }
 EXPORT_SYMBOL(scnprintf);