rbd: bail if too many arguments provided

rbd: bail if too many arguments provided

The code has a catch clause for that, but it was being rendered useless
by the preceding

    if (command_spec.size() > matching_spec->size())
      positional_options.add(at::POSITIONAL_ARGUMENTS.c_str(), -1);

which names all (both expected and extraneous) positional arguments.

Change it to name only expected arguments, deriving the number of
expected arguments from the length of positional_opts vector, supplied
by each action.  This works for all actions except "feature enable" and
"feature disable" which are specified as multitoken, so keep on passing
in -1 for those.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>