rcu: Remove return value from RCU_INIT_POINTER()

The return value from RCU_INIT_POINTER() is not used, and using it
would be quite ugly, for example:

	q = RCU_INIT_POINTER(global_p, p);

To prevent this sort of ugliness from appearing, this commit wraps
RCU_INIT_POINTER() in a do-while loop.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

include/linux/rcupdate.h

@@ -903,7 +903,9 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
  * the reader-accessible portions of the linked structure.
  */
 #define RCU_INIT_POINTER(p, v) \
-		p = (typeof(*v) __force __rcu *)(v)
+	do { \
+		p = (typeof(*v) __force __rcu *)(v); \
+	} while (0)
 
 /**
  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer