simple_write_end(): don't zero in short copy into uptodate

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/libfs.c

@@ -465,6 +465,8 @@ EXPORT_SYMBOL(simple_write_begin);
  * is not called, so a filesystem that actually does store data in .write_inode
  * should extend on what's done here with a call to mark_inode_dirty() in the
  * case that i_size has changed.
+ *
+ * Use *ONLY* with simple_readpage()
  */
 int simple_write_end(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len, unsigned copied,
@@ -474,14 +476,14 @@ int simple_write_end(struct file *file, struct address_space *mapping,
 	loff_t last_pos = pos + copied;
 
 	/* zero the stale part of the page if we did a short copy */
+	if (!PageUptodate(page)) {
 		if (copied < len) {
 			unsigned from = pos & (PAGE_SIZE - 1);
 
 			zero_user(page, from + copied, len - copied);
 		}
-
-	if (!PageUptodate(page))
 		SetPageUptodate(page);
+	}
 	/*
 	 * No need to use i_size_read() here, the i_size
 	 * cannot change under us because we hold the i_mutex.