powerpc: Add const qual to local_read() parameter

A patch in net-next triggered a compile error on powerpc:

  include/linux/u64_stats_sync.h: In function 'u64_stats_read':
  include/asm-generic/local64.h:30:37: warning: passing argument 1 of 'local_read' discards 'const' qualifier from pointer target type

This seems reasonable to relax powerpc local_read() requirements.

Fixes: 316580b ("u64_stats: provide u64_stats_t type")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au> # build only
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

arch/powerpc/include/asm/local.h

@@ -17,7 +17,7 @@ typedef struct
 
 #define LOCAL_INIT(i)	{ (i) }
 
-static __inline__ long local_read(local_t *l)
+static __inline__ long local_read(const local_t *l)
 {
 	return READ_ONCE(l->v);
 }