mdmon: record sync_completed directly to the metadata

mdmon: record sync_completed directly to the metadata

When sync_action is idle mdmon takes the latest value of md/resync_start
or md/<dev>/recovery_start to record the resync/rebuild checkpoint in
the metadata.  However, now that mdmon is reading sync_completed there
is no longer a need to wait for, or force an idle event to take a
checkpoint.

Simply update the forward progress of ->last_checkpoint at every wakeup
event and force it to be recorded at least every 1/16th array-size
interval.  It may be recorded more frequently if a ->set_array_state()
event occurs.

This also cleans up some confusion in handling the dual-rebuild case.
If more than one spare has been activated the kernel starts the rebuild
at the lowest recovery offset, so we do not need to worry about
min_recovery_start().

Signed-off-by: Dan Williams <dan.j.williams@intel.com>