powerpc: Use task_stack_page() in current_pt_regs()

Change current_pt_regs() to use task_stack_page() rather than
current_thread_info() so that it keeps working once we enable
THREAD_INFO_IN_TASK.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of large patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/ptrace.h

@@ -157,7 +157,7 @@ extern int ptrace_put_reg(struct task_struct *task, int regno,
 			  unsigned long data);
 
 #define current_pt_regs() \
-	((struct pt_regs *)((unsigned long)current_thread_info() + THREAD_SIZE) - 1)
+	((struct pt_regs *)((unsigned long)task_stack_page(current) + THREAD_SIZE) - 1)
 /*
  * We use the least-significant bit of the trap field to indicate
  * whether we have saved the full set of registers, or only a