afs: Fix afs_write_end() when called with copied == 0 [ver #2]

When afs_write_end() is called with copied == 0, it tries to set the dirty
region, but there's no way to actually encode a 0-length region in the
encoding in page->private.  "0,0", for example, indicates a 1-byte region
at offset 0.  The maths miscalculates this and sets it incorrectly.

Fix it to just do nothing but unlock and put the page in this case.  We
don't actually need to mark the page dirty as nothing presumably changed.

Fixes: 65dd2d6 ("afs: Alter dirty range encoding in page->private")
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/write.c

@@ -169,11 +169,14 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 	unsigned int f, from = pos & (PAGE_SIZE - 1);
 	unsigned int t, to = from + copied;
 	loff_t i_size, maybe_i_size;
-	int ret;
+	int ret = 0;
 
 	_enter("{%llx:%llu},{%lx}",
 	       vnode->fid.vid, vnode->fid.vnode, page->index);
 
+	if (copied == 0)
+		goto out;
+
 	maybe_i_size = pos + copied;
 
 	i_size = i_size_read(&vnode->vfs_inode);
@@ -196,7 +199,7 @@ int afs_write_end(struct file *file, struct address_space *mapping,
 			if (ret < 0)
 				goto out;
 		}
-		SetPageUptodate(page);
+		SetPageUptoodate(page);
 	}
 
 	if (PagePrivate(page)) {