[RBTREE] Add explicit alignment to sizeof(long) for struct rb_node.

Seems like a strange requirement, but allegedly it was necessary for
struct address_space on CRIS, because it otherwise ended up being only
byte-aligned. It's harmless enough, and easier to just do it than to
prove it isn't necessary... although I really ought to dig out my etrax
board and test it some time.
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

include/linux/rbtree.h

@@ -104,7 +104,8 @@ struct rb_node
 #define	RB_BLACK	1
 	struct rb_node *rb_right;
 	struct rb_node *rb_left;
-};
+} __attribute__((aligned(sizeof(long))));
+    /* The alignment might seem pointless, but allegedly CRIS needs it */
 
 struct rb_root
 {