1/ set new MD_RECOVERY_INTR flag instead of old 'err = -EINTR'
   when a resync thread is signaled - get rid of 'err' altogether in
   md_do_sync
2/ raid1 determines if resync is needed based on recovery_cp
   rather than mddev->in_sync (which now has a very different meaning)
3/ Don't update superblock when switching to writable mode.  The
   first write will update the superblock instead.