Remove hardcoded checkpoint interval checking

Remove hardcoded checkpoint interval checking

Mdmon assumes that kernel marks checkpoint every 1/16 of the volume size
and that the checkpoints are equal in size. This is not true, kernel may
mark checkpoints more frequently depending on several factors, including
sync speed. This results in checkpoints reported by mdadm --examine
falling behind the one reported by kernel.

Remove hardcoded checkpoint interval checking.

Signed-off-by: Mateusz Kusiak <mateusz.kusiak@intel.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>