rcu: add __rcu API for later sparse checking

This commit defines an __rcu API, but provides only vacuous definitions
for it.  This breaks dependencies among most of the subsequent patches,
allowing them to reach mainline asynchronously via whatever trees are
appropriate.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Christopher Li <sparse@chrisli.org>
Cc: Josh Triplett <josh@joshtriplett.org>

include/linux/compiler.h

@@ -16,6 +16,7 @@
 # define __release(x)	__context__(x,-1)
 # define __cond_lock(x,c)	((c) ? ({ __acquire(x); 1; }) : 0)
 # define __percpu	__attribute__((noderef, address_space(3)))
+# define __rcu
 extern void __chk_user_ptr(const volatile void __user *);
 extern void __chk_io_ptr(const volatile void __iomem *);
 #else
@@ -34,6 +35,7 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 # define __release(x) (void)0
 # define __cond_lock(x,c) (c)
 # define __percpu
+# define __rcu
 #endif
 
 #ifdef __KERNEL__