afs: Fix afs_write_end() when called with copied == 0 [ver #3]

[ Upstream commit 3ad216e ]

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

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);