Limit the maximum size of softraid crypto volumes that can be created to

Limit the maximum size of softraid crypto volumes that can be created to
the size that is currently supported (a data area of 16TB, or
2^32 * 32 + 528 blocks including the softraid metadata/boot area), rather
than successfully creating the volume and then failing during resource
allocation (without properly reporting why it failed).

Found the hard way by sharon s. <mymlact at gmx dot com>