xfs: introduce xfs_bulkstat_ichunk_ra

From: Jie Liu <jeff.liu@oracle.com>

Introduce xfs_bulkstat_ichunk_ra() to loop over all clusters in the
next inode chunk, then performs readahead if there are any allocated
inodes in that cluster.

Refactor xfs_bulkstat() with it.

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