LoongArch: Change __my_cpu_offset definition to avoid mis-optimization

From GCC commit 3f13154553f8546a ("df-scan: remove ad-hoc handling of
global regs in asms"), global registers will no longer be forced to add
to the def-use chain. Then current_thread_info(), current_stack_pointer
and __my_cpu_offset may be lifted out of the loop because they are no
longer treated as "volatile variables".

This optimization is still correct for the current_thread_info() and
current_stack_pointer usages because they are associated to a thread.
However it is wrong for __my_cpu_offset because it is associated to a
CPU rather than a thread: if the thread migrates to a different CPU in
the loop, __my_cpu_offset should be changed.

Change __my_cpu_offset definition to treat it as a "volatile variable",
in order to avoid such a mis-optimization.

Cc: stable@vger.kernel.org
Reported-by: Xiaotian Wu <wuxiaotian@loongson.cn>
Reported-by: Miao Wang <shankerwangmiao@gmail.com>
Signed-off-by: Xing Li <lixing@loongson.cn>
Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
Signed-off-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/percpu.h

@@ -29,7 +29,12 @@ static inline void set_my_cpu_offset(unsigned long off)
 	__my_cpu_offset = off;
 	csr_write64(off, PERCPU_BASE_KS);
 }
-#define __my_cpu_offset __my_cpu_offset
+
+#define __my_cpu_offset					\
+({							\
+	__asm__ __volatile__("":"+r"(__my_cpu_offset));	\
+	__my_cpu_offset;				\
+})
 
 #define PERCPU_OP(op, asm_op, c_op)					\
 static __always_inline unsigned long __percpu_##op(void *ptr,		\