nilfs2: use nilfs_btree_node_size()

Use nilfs_btree_node_size() instead of open-coding.

Link: http://lkml.kernel.org/r/1485184655-3895-2-git-send-email-konishi.ryusuke@lab.ntt.co.jp


Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>