Fix: consumer: fallback to flush when flush empty is unsupported

Session destruction fails on older (<= 2.8) lttng-modules as the
flush_empty fails on the kernel streams during the quiet rotation.

Fallback to the regular flush as the semantics of regular rotations
are not expected here; we merely want to flush any pending data and
destroy the session.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ifdf8a4e60b55dbf582747d71f5c2485d24c11964