riscv: Cleanup unnecessary define in asm-offset.c

 - TASK_THREAD_SP is duplicated define
 - TASK_STACK is no use at all
 - Don't worry about thread_info's offset in task_struct, have
   a look on comment in include/linux/sched.h:

struct task_struct {
	/*
	 * For reasons of header soup (see current_thread_info()), this
	 * must be the first element of task_struct.
	 */
	struct thread_info		thread_info;
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/asm-offsets.c

@@ -27,9 +27,6 @@ void asm_offsets(void)
 	OFFSET(TASK_THREAD_S9, task_struct, thread.s[9]);
 	OFFSET(TASK_THREAD_S10, task_struct, thread.s[10]);
 	OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
-	OFFSET(TASK_THREAD_SP, task_struct, thread.sp);
-	OFFSET(TASK_STACK, task_struct, stack);
-	OFFSET(TASK_TI, task_struct, thread_info);
 	OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);

arch/riscv/kernel/entry.S

@@ -435,12 +435,8 @@ ENTRY(__switch_to)
 	lw a4, TASK_TI_CPU(a1)
 	sw a3, TASK_TI_CPU(a1)
 	sw a4, TASK_TI_CPU(a0)
-#if TASK_TI != 0
-#error "TASK_TI != 0: tp will contain a 'struct thread_info', not a 'struct task_struct' so get_current() won't work."
-	addi tp, a1, TASK_TI
-#else
+	/* The offset of thread_info in task_struct is zero. */
 	move tp, a1
-#endif
 	ret
 ENDPROC(__switch_to)