nilfs2: separate constructor of metadata files

This will displace nilfs_mdt_new() constructor with individual
metadata file constructors like nilfs_dat_new(), new_sufile_new(),
nilfs_cpfile_new(), and nilfs_ifile_new().

This makes it possible for each metadata file to have own
intialization code.

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

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

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

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

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

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

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

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

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

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

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