ceph: return -EINVAL if given fsc mount option on kernel w/o support

If someone requests fscache on the mount, and the kernel doesn't
support it, it should fail the mount.

[ Drop ceph prefix -- it's provided by pr_err. ]

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