Fix: Implement RING_BUFFER_GET_NEXT_SUBBUF_METADATA_CHECK

Fix: Implement RING_BUFFER_GET_NEXT_SUBBUF_METADATA_CHECK

Get next metadata subbuffer, returning a flag indicating whether the
metadata is guaranteed to be in a consistent state at the end of this
sub-buffer (can be parsed).

This can be used by the consumer to know whether the metadata can be
parsed at the end of this sub-buffer, which is useful to distinguish
between errors and incomplete metadata in live tracing.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>