s390/keyboard: sanitize array index in do_kdsk_ioctl

s390/keyboard: sanitize array index in do_kdsk_ioctl

The kbd_ioctl uses two user controlled indexes for KDGKBENT/KDSKBENT.
Use array_index_nospec to prevent any out of bounds speculation.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
CVE-2017-5753

(cherry picked from commit 054732837c312c6007994badbc86b0976b228f77)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>