xfs: Don't issue buffer IO direct from AIL push V2

All buffers logged into the AIL are marked as delayed write.
When the AIL needs to push the buffer out, it issues an async write of the
buffer. This means that IO patterns are dependent on the order of
buffers in the AIL.

Instead of flushing the buffer, promote the buffer in the delayed
write list so that the next time the xfsbufd is run the buffer will
be flushed by the xfsbufd. Return the state to the xfsaild that the
buffer was promoted so that the xfsaild knows that it needs to cause
the xfsbufd to run to flush the buffers that were promoted.

Using the xfsbufd for issuing the IO allows us to dispatch all
buffer IO from the one queue. This means that we can make much more
enlightened decisions on what order to flush buffers to disk as
we don't have multiple places issuing IO. Optimisations to xfsbufd
will be in a future patch.

Version 2
- kill XFS_ITEM_FLUSHING as it is now unused.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_inode_item.h

@@ -144,12 +144,6 @@ typedef struct xfs_inode_log_item {
 						      data exts */
 	struct xfs_bmbt_rec	*ili_aextents_buf; /* array of logged
 						      attr exts */
-	unsigned int            ili_pushbuf_flag;  /* one bit used in push_ail */
-
-#ifdef DEBUG
-	uint64_t                ili_push_owner;    /* one who sets pushbuf_flag
-						      above gets to push the buf */
-#endif
 #ifdef XFS_TRANS_DEBUG
 	int			ili_root_size;
 	char			*ili_orig_root;