bcachefs: Convert open-coded extra computation to helper

This patch replaces open-coded extra computation to eytzinger1_extra.
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bset.c

@@ -732,7 +732,7 @@ static noinline void __build_ro_aux_tree(struct btree *b, struct bset_tree *t)
 		return;
 	}
 
-	t->extra = (t->size - rounddown_pow_of_two(t->size - 1)) << 1;
+	t->extra = eytzinger1_extra(t->size - 1);
 
 	/* First we figure out where the first key in each cacheline is */
 	eytzinger1_for_each(j, t->size - 1) {