mmap.c corner case fix, per David S. Miller

Hi Marcelo :)

This patch fixes the same corner case, but does something useful
even or architectures where TASK_SIZE is greater than SIZE_MAX-PAGE_SIZE

The patch is from David S. Miller, not me. My patch was
incomplete and did nothing on 'big TASK_SIZE' architectures, as sparc64.

The patch is against both 2.4.20 and 2.4.21-pre1, is just the same.

Raúl

@@ -403,10 +403,12 @@
 	if (file && (!file->f_op || !file->f_op->mmap))
 		return -ENODEV;
 
-	if ((len = PAGE_ALIGN(len)) == 0)
+	if (!len)
 		return addr;
 
-	if (len > TASK_SIZE)
+	len = PAGE_ALIGN(len);
+
+	if (len > TASK_SIZE || len == 0)
 		return -EINVAL;
 
 	/* offset overflow? */