xfs: remove redundant user buffer count checks at xfs_bulkstat

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

Remove the redundant user buffer and count checks as it has already
been validated at xfs_ioc_bulkstat().

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