rcu: Add a gcc-style structure initializer for RCU pointers

RCU_INIT_POINTER() returns a value that is never used, and which should
be abolished due to terminal ugliness:

	q = RCU_INIT_POINTER(global_p, p);

However, there are two uses that cannot be handled by a do-while
formulation because they do gcc-style initialization:

	RCU_INIT_POINTER(.real_cred, &init_cred),
	RCU_INIT_POINTER(.cred, &init_cred),

This usage is clever, but not necessarily the nicest approach.
This commit therefore creates an RCU_POINTER_INITIALIZER() macro that
is specifically designed for gcc-style initialization.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>

include/linux/rcupdate.h

@@ -905,6 +905,14 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
 #define RCU_INIT_POINTER(p, v) \
 		p = (typeof(*v) __force __rcu *)(v)
 
+/**
+ * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
+ *
+ * GCC-style initialization for an RCU-protected pointer in a structure field.
+ */
+#define RCU_POINTER_INITIALIZER(p, v) \
+		.p = (typeof(*v) __force __rcu *)(v)
+
 static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
 {
 	return offset < 4096;