nilfs2: allow future expansion of metadata read out via get info ioctl

Nilfs has some ioctl commands to read out metadata from meta data
files:

 - NILFS_IOCTL_GET_CPINFO for checkpoint file,
 - NILFS_IOCTL_GET_SUINFO for segment usage file, and
 - NILFS_IOCTL_GET_VINFO for Disk Address Transalation (DAT) file,
   respectively.

Every routine on these metadata files is implemented so that it allows
future expansion of on-disk format.  But, the above ioctl commands do
not support expansion even though nilfs_argv structure can handle
arbitrary size for data exchanged via ioctl.

This allows future expansion of the following structures which give
basic format of the "get information" ioctls:

 - struct nilfs_cpinfo
 - struct nilfs_suinfo
 - struct nilfs_vinfo

So, this introduces forward compatility of such ioctl commands.

In this patch, a sanity check in nilfs_ioctl_get_info() function is
changed to accept larger data structure [1], and metadata read
routines are rewritten so that they become compatible for larger
structures; the routines will just ignore the remaining fields which
the current version of nilfs doesn't know.

[1] The ioctl function already has another upper limit (PAGE_SIZE
    against a structure, which appears in nilfs_ioctl_wrap_copy
    function), and this will not cause security problem.

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

fs/nilfs2/dat.h

@@ -47,6 +47,6 @@ void nilfs_dat_abort_end(struct inode *, struct nilfs_palloc_req *);
 int nilfs_dat_mark_dirty(struct inode *, __u64);
 int nilfs_dat_freev(struct inode *, __u64 *, size_t);
 int nilfs_dat_move(struct inode *, __u64, sector_t);
-ssize_t nilfs_dat_get_vinfo(struct inode *, struct nilfs_vinfo *, size_t);
+ssize_t nilfs_dat_get_vinfo(struct inode *, void *, unsigned, size_t);
 
 #endif	/* _NILFS_DAT_H */