writeback: fix time ordering of the per superblock dirty inode lists 3

While writeback is working against a dirty inode it does a check after trying
to write some of the inode's pages:

"did the lower layers skip some of the inode's dirty pages because they were
locked (or under writeback, or whatever)"

If this turns out to be true, we must move the inode back onto s_dirty and
redirty it.  The reason for doing this is that fsync() and friends only check
the s_dirty list, and those functions want to know about those pages which
were locked, so they can be waited upon and, if necessary, rewritten.

Problem is, that redirtying was putting the inode onto the tail of s_dirty
without updating its timestamp.  This causes a violation of s_dirty ordering.

Fix this by updating inode->dirtied_when when moving the inode onto s_dirty.

But the code is still a bit buggy?  If the inode was _already_ dirty then we
don't need to move it at all.  Oh well, hopefully it doesn't matter too much,
as that was a redirtying, which was very recent anwyay.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -413,7 +413,7 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
 			 * writeback is not making progress due to locked
 			 * buffers.  Skip this inode for now.
 			 */
-			list_move(&inode->i_list, &sb->s_dirty);
+			redirty_tail(inode);
 		}
 		spin_unlock(&inode_lock);
 		iput(inode);