s390/mm: limit STACK_RND_MASK for compat tasks

For compat tasks the mmap randomization does not use the maximum
randomization value from mmap_rnd_mask but the fixed value of 0x7ff.
This needs to be respected in the definition of STACK_RND_MASK as
well.

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