s390/uaccess: add "fallthrough" comments

Add "fallthrough" comments so nobody wonders if a break statement is missing.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>