writeback: fix time ordering of the per superblock dirty inode lists 7

This one fixes four bugs.

There are a few situation in there where writeback decides it is going to skip
over a blockdev inode on the kernel-internal blockdev superblock.  It
presently does this by moving the blockdev inode onto the tail of the blockdev
superblock's s_dirty.  But

a) this screws up s_dirty's reverse-time-orderedness and

b) refiling the blockdev for writeback in another 30 second is rude.  We
   should try again sooner than that.

Fix all this up by using redirty_head(): move the blockdev inode onto the head
of the blockdev superblock's s_dirty list for prompt writeback.

Cc: Mike Waychison <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/fs-writeback.c

@@ -410,14 +410,14 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
 			wbc->encountered_congestion = 1;
 			if (!sb_is_blkdev_sb(sb))
 				break;		/* Skip a congested fs */
-			list_move(&inode->i_list, &sb->s_dirty);
+			redirty_head(inode);
 			continue;		/* Skip a congested blockdev */
 		}
 
 		if (wbc->bdi && bdi != wbc->bdi) {
 			if (!sb_is_blkdev_sb(sb))
 				break;		/* fs has the wrong queue */
-			list_move(&inode->i_list, &sb->s_dirty);
+			redirty_head(inode);
 			continue;		/* blockdev has wrong queue */
 		}