xfs: properly type the b_log_item field in struct xfs_buf

Now that the log code doesn't abuse this field any more we can
declare it as a struct xfs_buf_log_item pointer.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_buf.h

@@ -150,7 +150,7 @@ typedef struct xfs_buf {
 	struct work_struct	b_ioend_work;
 	xfs_buf_iodone_t	b_iodone;	/* I/O completion function */
 	struct completion	b_iowait;	/* queue for I/O waiters */
-	void			*b_log_item;
+	struct xfs_buf_log_item	*b_log_item;
 	struct list_head	b_li_list;	/* Log items list head */
 	struct xfs_trans	*b_transp;
 	struct page		**b_pages;	/* array of page pointers */