nilfs2: add size option of private object to metadata file allocator

This adds an optional "object size" argument to nilfs_mdt_new_common()
function; the argument specifies the size of private object attached
to a newly allocated metadata file inode.

This will afford space to keep local variables for meta data files.

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

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

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

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

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

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