[PATCH -tip] ring_buffer: fix warning

[Lai Jiangshan <laijs@cn.fujitsu.com>]

kernel/trace/ring_buffer.c: In function 'rb_tail_page_update':
kernel/trace/ring_buffer.c:849: warning: value computed is not used
kernel/trace/ring_buffer.c:850: warning: value computed is not used

Add a "(void)" to fix this warning.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index e648ba4..d371279 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -846,8 +846,8 @@ static int rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
 		 * not come in and change it. In which case, we
 		 * do not want to modify it.
 		 */
-		local_cmpxchg(&next_page->write, old_write, val);
-		local_cmpxchg(&next_page->entries, old_entries, eval);
+		(void)local_cmpxchg(&next_page->write, old_write, val);
+		(void)local_cmpxchg(&next_page->entries, old_entries, eval);
 
 		/*
 		 * No need to worry about races with clearing out the commit.