s390/keyboard: avoid off-by-one when using strnlen_user()

strnlen_user() returns the length of the string including terminating 0.
So avoid counting it again and unnecessarily reducing maximum string
size by 1.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>