ceph: rename _to_client() to _to_fs_client()

We need to covert the inode to ceph_client in the following commit,
and will add one new helper for that, here we rename the old helper
to _fs_client().

Link: https://tracker.ceph.com/issues/61590


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>