Fix: allow get/put subbuf in loop for metadata stream

data_pending check can trigger the following sequence:

- get_next_subbuf
- put_subbuf
- get_next_subbuf
- put_subbuf
- get_next_subbuf
- put_subbuf
...

and then finally a thread would consume the data:

- get_next_subbuf
- put_next_subbuf

However, we don't want to populate data from the metadata cache into the
stream until put_next_subbuf is issued. Add a check to ensure that it is
not populated until required.

Also, disallow get_subbuf() ioctl on metadata channel: its random-access
semantic does not play well with serialization of the metadata cache on
demand.

Reviewed-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>