mdadm: failed to add /dev/md127 to /dev/md0: Invalid argument
mdadm: add new device failed for /dev/md127 as 4: Invalid argument
@Gem, While it is nice to set a high data_offset to leave plenty of head
room it is much more important to leave enough space to allow
of the data of the array.
So after we check that sb->size is still available, only reduce the
'reserved', don't increase it.
This fixes a bug where --adding a spare fails because it does not have
enough space in it.