powerpc: Check that TASK_SIZE does not overlap KERNEL_START

Make sure CONFIG_TASK_SIZE does not overlap CONFIG_KERNEL_START
This could happen when overriding settings to get 1GB lowmem, and would lead
to userland mysteriousely hanging.

This setting is only used by PPC32.

Signed-off-by: Rune Torgersen <runet@innovsys.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/processor.h

@@ -81,6 +81,10 @@ extern struct task_struct *last_task_used_altivec;
 extern struct task_struct *last_task_used_spe;
 
 #ifdef CONFIG_PPC32
+
+#if CONFIG_TASK_SIZE > CONFIG_KERNEL_START
+#error User TASK_SIZE overlaps with KERNEL_START address
+#endif
 #define TASK_SIZE	(CONFIG_TASK_SIZE)
 
 /* This decides where the kernel will search for a free chunk of vm