s390/traps: panic() instead of die() on translation exception

In case of a translation exception the page tables are corrupted. If the
exception handler then calls die() which again calls show_regs()
-> show_code() -> copy_from_user(), the kernel may access the same memory
location again and generates yet another translation exception. Which in turn
will lead to a deadlock on the die_lock spinlock, which the kernel tries to
grab recursively.

Given that the page tables are corrupted anyway, if we see such an exception,
let's simply panic.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>