ceph: move net/ceph/ceph_fs.c to fs/ceph/util.c

All of these functions are only called from CephFS, so move them into
ceph.ko, and drop the exports.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>