ceph: add new field max_file_size in ceph_fs_client

In order to not bother to VFS and other specific filesystems,
we decided to do offset validation inside ceph kernel client,
so just simply set sb->s_maxbytes to MAX_LFS_FILESIZE so that
it can successfully pass VFS check. We add new field max_file_size
in ceph_fs_client to store real file size limit and doing proper
check based on it.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>