fix detection of non-LUKS CRYPT

fix detection of non-LUKS CRYPT

grub_util_get_dm_abstraction() does a string comparison of insufficient
length. When using a UUID such as "CRYPT-PLAIN-sda6_crypt", the function
returns GRUB_DEV_ABSTRACTION_LUKS.

This results in the error:
    ./grub-probe: error: disk `cryptouuid/sda6_crypt' not found.

This appears to be a copy/paste error introduced in:
a10e7a5a8918bea6e2632055129fa9b516fe965a

The bug was (apparently) latent until revealed by:
3bca85b4184f74995a7cc2791e432173fde26d34

Signed-off-by: Corey Hickey <bugfood-c@fatooh.org>