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>

arch/s390/include/asm/elf.h

@@ -211,7 +211,7 @@ do {								\
 
 extern unsigned long mmap_rnd_mask;
 
-#define STACK_RND_MASK	(mmap_rnd_mask)
+#define STACK_RND_MASK	(test_thread_flag(TIF_31BIT) ? 0x7ff : mmap_rnd_mask)
 
 #define ARCH_DLINFO							    \
 do {									    \