libceph: report defined but unsupported osd ops

If osd_req_encode_op() is given any opcode it doesn't recognize
it reports an error.

This patch fleshes out that routine to distinguish between
well-defined but unsupported values and values that are simply
bogus.

This and the next commit are related to:
    http://tracker.ceph.com/issues/4126

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>