ceph: switch to WARN_ON_ONCE in encode_supported_features()

...and let the errnos bubble up to the callers.

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