percpu: use __weak only in the definition of weak percpu variables

__weak is necessary only for definition and might even not work in
declaration.  Drop it from declaration.

This change was suggested by Ivan Kokshaysky.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>

include/linux/percpu-defs.h

@@ -56,7 +56,7 @@
  */
 #define DECLARE_PER_CPU_SECTION(type, name, sec)			\
 	extern __PCPU_DUMMY_ATTRS char __pcpu_scope_##name;		\
-	extern __PCPU_ATTRS(sec) __weak __typeof__(type) per_cpu__##name
+	extern __PCPU_ATTRS(sec) __typeof__(type) per_cpu__##name
 
 #define DEFINE_PER_CPU_SECTION(type, name, sec)				\
 	__PCPU_DUMMY_ATTRS char __pcpu_scope_##name;			\