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>