stabd/geli: Bail out if you can't get the disks size

If the DIOCGMEDIASIZE ioctl fails, assume the disk doesn't have geli
encryption. While all disks should implement this, fail safe for disks /
partitions that do not.

Sponsored by: Netflix

(cherry picked from commit 787df454c8175e58131f582c05c169070fb6ca7c)