nilfs2: implement FS_IOC_GETFLAGS/SETFLAGS/GETVERSION

Add support for the standard attributes set via chattr and read via
lsattr.  These attributes are already in the flags value in the nilfs2
inode, but currently we don't have any ioctl commands that expose them
to the userland.

Collaterally, this adds the FS_IOC_GETVERSION ioctl for getting
i_generation, which allows users to list the file's generation number
with "lsattr -v".

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