xfs: introduce xfs_bulkstat_ag_ichunk

Introduce xfs_bulkstat_ag_ichunk() to process inodes in chunk with a
pointer to a formatter function that will iget the inode and fill in
the appropriate structure.

Refactor xfs_bulkstat() with it.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>