nilfs2: move definition of struct nilfs_btree_node

This is a trivial patch to expose struct nilfs_fs_btree_node.
The struct should be exposed outside of kernel, for it is disk format.

Signed-off-by: Jiro SEKIBA <jir@unicus.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>