luks2: Do not handle disks of size GRUB_DISK_SIZE_UNKNOWN for now

luks2: Do not handle disks of size GRUB_DISK_SIZE_UNKNOWN for now

Check to make sure that source disk has a known size. If not, print
a message and return error. There are 4 cases where GRUB_DISK_SIZE_UNKNOWN
is set (biosdisk, obdisk, ofdisk, and uboot), and in all those cases
processing continues. So this is probably a bit conservative. However,
3 of the cases seem pathological, and the other, biosdisk, happens when
booting from a CD-ROM. Since I doubt booting from a LUKS2 volume on
a CD-ROM is a big use case, we'll error until someone complains.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>