kernel/smp.c: use '|=' for csd_lock

csd_lock() uses assignment to data->flags rather than |=.  That is not
buggy at present because only one bit (CSD_FLAG_LOCK) is defined in
call_single_data.flags.

But it will become buggy if we later add another flag, so fix it now.
Signed-off-by: Nliguang <lig.fnst@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/smp.c

@@ -109,7 +109,7 @@ static void csd_lock_wait(struct call_single_data *data)
 static void csd_lock(struct call_single_data *data)
 {
 	csd_lock_wait(data);
-	data->flags = CSD_FLAG_LOCK;
+	data->flags |= CSD_FLAG_LOCK;
 
 	/*
 	 * prevent CPU from reordering the above assignment