xfs: remove xfs_bulkstat_single

From: Christoph Hellwig <hch@lst.de>

xfs_bukstat_one doesn't have any failure case that would go away when
called through xfs_bulkstat, so remove the fallback and the now unessecary
xfs_bulkstat_single function.

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