1/ Personalities only know about raid_disks devices.
   Some might be not in_sync and so cannot be read from,
   but must be written to.
	- change MD_SB_DISKS to ->raid_disks
	- add tests for .write_only

2/ rdev->raid_disk is now -1 for spares.  desc_nr is maintained
   by analyse_sbs and sync_sbs.

3/ spare_inactive method is subsumed into hot_remove_disk
   spare_writable is subsumed into hot_add_disk.
   hot_add_disk decides which slot a new device will hold.

4/ spare_active now finds all non-in_sync devices and marks them
   in_sync.

5/ faulty devices are removed by the md recovery thread as soon
   as they are idle.  Any spares that are available are then added.