documentation: register ioctl entry of nilfs2

This will register the ioctl range used by nilfs2 file system to the
table listed in Documentation/ioctl/ioctl-number.txt.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>