nilfs2: always set back pointer to host inode in mapping->host

In the current nilfs, page cache for btree nodes and meta data files
do not set a valid back pointer to the host inode in mapping->host.

This will change it so that every address space in nilfs uses
mapping->host to hold its host inode.

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

diff --cc fs/nilfs2/btnode.c
Simple merge

diff --cc fs/nilfs2/btnode.h
Simple merge

diff --cc fs/nilfs2/mdt.c
Simple merge

diff --cc fs/nilfs2/nilfs.h
Simple merge

diff --cc fs/nilfs2/page.c
Simple merge

diff --cc fs/nilfs2/page.h
Simple merge

diff --cc fs/nilfs2/segment.c
Simple merge

diff --cc fs/nilfs2/super.c
Simple merge