super1: leave more space in front of data by default.

super1: leave more space in front of data by default.

The kernel is growing the ability to avoid the need for a
backup file during reshape by being able to change the data offset.

For this to be useful we need plenty of free space before the
data so the data offset can be reduced.

So for v1.1 and v1.2 metadata make the default data_offset much
larger.  Aim for 128Meg, but keep a power of 2 and don't use more
than 0.1% of each device.

Don't change v1.0 as that is used when the data_offset is required to
be zero.

Signed-off-by: NeilBrown <neilb@suse.de>