radix-tree: Fix __rcu annotations

Many places were missing __rcu annotations.  A few places needed a few
lines of explanation about why it was safe to not use RCU accessors.
Add a custom CFLAGS setting to the Makefile to ensure that new patches
don't miss RCU annotations.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

lib/Makefile

@@ -24,6 +24,8 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
 	 earlycpio.o seq_buf.o nmi_backtrace.o nodemask.o win_minmax.o
 
+CFLAGS_radix-tree.o += -DCONFIG_SPARSE_RCU_POINTER
+
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o
 lib-$(CONFIG_HAS_DMA) += dma-noop.o