Md devices (raid1/raid5) can resync or recover.
There are similar but importantly different.
resync happens after an unclean shutdown
recovery happens when a failed drive is being replaced by a hot spare.

The sync-checkpoint code confused the two somewhat and this causes
problems.

This patch makes sure "recovery_cp" only relates to resync,
not recovery.
It also fixes a small problem with recording spares in
the superblock.