f2fs: clean up structure extent_node

The union in struct extent_node wass only to indicate below fields

	struct rb_node rb_node;
	union {
		struct {
			unsigned int fofs;
			unsigned int len;
		...
	...

can be parsed as fields in struct rb_entry, but they were never be
used explicitly before, so let's remove them for cleanup.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>