ring-buffer: typecast cmpxchg to fix PowerPC warning

The cmpxchg used by PowerPC does the following:

  ({									 \
     __typeof__(*(ptr)) _o_ = (o);					 \
     __typeof__(*(ptr)) _n_ = (n);					 \
     (__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_,		 \
				    (unsigned long)_n_, sizeof(*(ptr))); \
  })

This does a type check of *ptr to both o and n.

Unfortunately, the code in ring-buffer.c assigns longs to pointers
and pointers to longs and causes a warning on PowerPC:

ring_buffer.c: In function 'rb_head_page_set':
ring_buffer.c:704: warning: initialization makes pointer from integer without a cast
ring_buffer.c:704: warning: initialization makes pointer from integer without a cast
ring_buffer.c: In function 'rb_head_page_replace':
ring_buffer.c:797: warning: initialization makes integer from pointer without a cast

This patch adds the typecasts inside cmpxchg to annotate that a long is
being cast to a pointer and a pointer is being casted to a long and this
removes the PowerPC warnings.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -701,8 +701,8 @@ static int rb_head_page_set(struct ring_buffer_per_cpu *cpu_buffer,
 
 	val &= ~RB_FLAG_MASK;
 
-	ret = (unsigned long)cmpxchg(&list->next,
-				     val | old_flag, val | new_flag);
+	ret = cmpxchg((unsigned long *)&list->next,
+		      val | old_flag, val | new_flag);
 
 	/* check if the reader took the page */
 	if ((ret & ~RB_FLAG_MASK) != val)
@@ -794,7 +794,7 @@ static int rb_head_page_replace(struct buffer_page *old,
 	val = *ptr & ~RB_FLAG_MASK;
 	val |= RB_PAGE_HEAD;
 
-	ret = cmpxchg(ptr, val, &new->list);
+	ret = cmpxchg(ptr, val, (unsigned long)&new->list);
 
 	return ret == val;
 }