MIPS: VDSO: Reduce VDSO_RANDOMIZE_SIZE to 64MB for 64bit

Commit ea7e0480 ("MIPS: VDSO: Always map near top of user memory")
set VDSO_RANDOMIZE_SIZE to 256MB for 64bit kernel. But take a look at
arch/mips/mm/mmap.c we can see that MIN_GAP is 128MB, which means the
mmap_base may be at (user_address_top - 128MB). This make the stack be
surrounded by mmaped areas, then stack expanding fails and causes a
segmentation fault. Therefore, VDSO_RANDOMIZE_SIZE should be less than
MIN_GAP and this patch reduce it to 64MB.
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: ea7e0480 ("MIPS: VDSO: Always map near top of user memory")
Patchwork: https://patchwork.linux-mips.org/patch/20910/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: Huacai Chen <chenhuacai@gmail.com>

arch/mips/include/asm/processor.h

@@ -81,7 +81,7 @@ extern unsigned int vced_count, vcei_count;
 
 #endif
 
-#define VDSO_RANDOMIZE_SIZE	(TASK_IS_32BIT_ADDR ? SZ_1M : SZ_256M)
+#define VDSO_RANDOMIZE_SIZE	(TASK_IS_32BIT_ADDR ? SZ_1M : SZ_64M)
 
 extern unsigned long mips_stack_top(void);
 #define STACK_TOP		mips_stack_top()