s390/keyboard: use memdup_user_nul()

Use memdup_user_nul to duplicate a memory region from user-space
to kernel-space and terminate with a NULL, instead of open coding
using kmalloc + copy_from_user and explicitly NULL terminating.

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
[heiko.carstens@de.ibm.com: remove comment]
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

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