nilfs2: embed a back pointer to super block instance in nilfs object

nilfs2: embed a back pointer to super block instance in nilfs object

Insert a back pointer to super block instance in nilfs object so that
functions of nilfs2 easily refer to the super block instance.  This
simplifies replacement of printk() in the successive change.

Link: http://lkml.kernel.org/r/1464875891-5443-4-git-send-email-konishi.ryusuke@lab.ntt.co.jp
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>