Fix symlink problem with newer device mapper, patch based on work by Thomas Koehler.

Original commit message by Thomas Koehler:

 Newer device mappers create /dev/mapper/grml-crypt_DEV
 as a symlink to /dev/dm-X, no longer as a block device.
 Unfortunately, after mounting, you only see /dev/dm-X in
 /etc/mtab and /proc/mounts, which broke the "stop" command
 in grml-crypt. By looping over all /dev/mapper/grml-crypt*
 files and checking if this is the correct one, this problem
 can be fixed.

Thanks: Thomas Koehler <jean-luc@picard.franken.de>