Manage: remove call to validate_geometry.

Manage: remove call to validate_geometry.

This call to validate_geometry is really rather gratuitous.
It is purely about the fact that super0 cannot use more than 4TB.
So just make it an explicit test - less confusing that way.

With this, validate_geometry is only called from Create, which
makes it easier to reason about.

Also validate_geometry is now never passed NULL for the 'chunk'
parameter, so we can remove those annoying tests for NULL.

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