s390/traps: improve panic message for translation-specification exception

There are many different types of translation exceptions but only a
translation-specification exception leads to a kernel panic since it
indicates corrupted page tables, which must never happen.

Improve the panic message so it is a bit more obvious what this is about.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>