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

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>