KEYS: output last portion of fingerprint in /proc/keys

Previous version of KEYS used to output last 4 bytes of fingerprint.
Now it outputs 8 last bytes of raw subject, which does not make any
visual meaning at all. This patch restores old behavior.

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: David Howells <dhowells@redhat.com>