[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: David Woodhouse <dwmw2@infradead.org>