add pre-io callback to struct xfs_buf

Index: linux-2.6-xfs/fs/xfs/linux-2.6/xfs_buf.c
===================================================================
@@ -1179,6 +1179,14 @@ _xfs_buf_ioapply(
 		     (bp->b_flags & XBF_READ_AHEAD) ? READA : READ;
 	}
 
+	/*
+	 * call out to buffer specific pre-write I/O functions. Used for
+	 * validation of buffers and CRC calculations prior to I/O issue.
+	 */
+	if (bp->b_io_callback && (bp->b_flags & XBF_WRITE))
+		bp->b_io_callback(bp);
+
+
 	/* Special code path for reading a sub page size buffer in --
 	 * we populate up the whole page, and hence the other metadata
 	 * in the same page.  This optimization is only valid when the

Index: linux-2.6-xfs/fs/xfs/linux-2.6/xfs_buf.h
===================================================================
@@ -166,6 +166,8 @@ typedef struct xfs_buf {
 	unsigned int		b_offset;	/* page offset in first page */
 	struct page		**b_pages;	/* array of page pointers */
 	struct page		*b_page_array[XB_PAGES]; /* inline pages */
+	void			(*b_io_callback)(struct xfs_buf *);
+						/* pre-write I/O callback */
 #ifdef XFS_BUF_LOCK_TRACKING
 	int			b_last_holder;
 #endif

@@ -228,6 +230,16 @@ static inline int xfs_buf_geterror(xfs_b
 /* Buffer Utility Routines */
 extern xfs_caddr_t xfs_buf_offset(xfs_buf_t *, size_t);
 
+/*
+ * Set the function that should be called immediately prior
+ * to a write I/O being issued on this buffer.
+ */
+static inline void
+xfs_buf_set_io_callback(xfs_buf_t *bp, void (*func)(xfs_buf_t *))
+{
+	bp->b_io_callback = func;
+}
+
 /* Pinning Buffer Storage in Memory */
 extern void xfs_buf_pin(xfs_buf_t *);
 extern void xfs_buf_unpin(xfs_buf_t *);