!9479 xfs: fix unlink vs cluster buffer instantiation race

		 * This buffer may not have been correctly initialised as we

		 * didn't read it from disk. That's not important because we are

		 * only using to mark the buffer as stale in the log, and to

		 * attach stale cached inodes on it. That means it will never be

		 * dispatched for IO. If it is, we want to know about it, and we

		 * want it to fail. We can acheive this by adding a write

		 * verifier to the buffer.

		 */

		 * attach stale cached inodes on it.

		 *

		 * For the inode that triggered the cluster freeing, this

		 * attachment may occur in xfs_inode_item_precommit() after we

		 * have marked this buffer stale.  If this buffer was not in

		 * memory before xfs_ifree_cluster() started, it will not be

		 * marked XBF_DONE and this will cause problems later in

		 * xfs_inode_item_precommit() when we trip over a (stale, !done)

		 * buffer to attached to the transaction.

		 *

		 * Hence we have to mark the buffer as XFS_DONE here. This is

		 * safe because we are also marking the buffer as XBF_STALE and

		 * XFS_BLI_STALE. That means it will never be dispatched for

		 * IO and it won't be unlocked until the cluster freeing has

		 * been committed to the journal and the buffer unpinned. If it

		 * is written, we want to know about it, and we want it to

		 * fail. We can acheive this by adding a write verifier to the

		 * buffer.

		 */

		bp->b_flags |= XBF_DONE;

		bp->b_ops = &xfs_inode_buf_ops;

		/*