s390/cmpxchg: select HAVE_CMPXCHG_LOCAL option

Now that hopefully all cmpxchg/xchg bugs have been fixed select
HAVE_CMPXCHG_LOCAL option which uncovered a couple of bugs on s390.

The only call site which is affected seems to be within mm/vmstat.c.

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