1/ don't free the rdev->sb on an error -- it might be
   accessed again later.  Just wait for the device to be
   exported.
2/ Change md_update_sb to __md_update_sb and have it
   clear the sb_dirty flag.
   New md_update_sb locks the device and calls __md_update_sb
   if sb_dirty.  This avoids any possbile races around
   updating the superblock