super1: fix choice of data_offset.

super1: fix choice of data_offset.

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.

Reported-by: nowhere <nowhere@hakkenden.ath.cx>
Signed-off-by: NeilBrown <neilb@suse.de>