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.jpSigned-off-by: NGeliang Tang <geliangtang@gmail.com>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>