[PATCH -next] compiler.h: fix compile error with gcc-8 or lower version

[Yang Yingliang <yangyingliang@xxxxxxxxxx>]

I got the follow compile error in latest linux-next kernel with gcc-8.4:

  In file included from <command-line>:
  ./include/linux/percpu.h: In function ‘__alloc_reserved_percpu’:
  ././include/linux/compiler_types.h:279:30: error: expected declaration specifiers before ‘__alloc_size__’
   #define __alloc_size(x, ...) __alloc_size__(x, ## __VA_ARGS__) __malloc
                                ^~~~~~~~~~~~~~
  ./include/linux/percpu.h:120:74: note: in expansion of macro ‘__alloc_size’
   extern void __percpu *__alloc_reserved_percpu(size_t size, size_t align) __alloc_size(1);

__alloc_size is not defined in gcc-8 or lower version, so add back the
conditional test for __alloc_size__ to fix it.

Fixes: 63caa04ec605 ("slab: Remove __malloc attribute from realloc functions")
Signed-off-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
---
 include/linux/compiler_types.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index aa81da7a28eb..aae8852ccbdc 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -276,8 +276,13 @@ struct ftrace_likely_data {
  * be performing a _reallocation_, as that may alias the existing pointer.
  * For these, use __realloc_size().
  */
+#ifdef __alloc_size__
 #define __alloc_size(x, ...)	__alloc_size__(x, ## __VA_ARGS__) __malloc
-#define __realloc_size(x, ...)	__alloc_size__(x, ## __VA_ARGS__)
+#define __realloc_size(x, ...) __alloc_size__(x, ## __VA_ARGS__)
+#else
+#define __alloc_size(x, ...)	__malloc
+#define __realloc_size(x, ...) __malloc
+#endif
 
 #ifndef asm_volatile_goto
 #define asm_volatile_goto(x...) asm goto(x)
-- 
2.25.1