bcachefs: implement FS_IOC_GETVERSION to support lsattr

In this patch we add the FS_IOC_GETVERSION ioctl for getting
i_generation from inode, after that, users can list file's
generation number by using "lsattr".

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>