Fix: metadata lttng channel refcount

The OOPS at bug #622 is likely caused by a missing reference on the
lttng channel structure, which could lead to accessing the object after
it has been destroyed if the lttng channel file descriptor is closed
while the metadata stream fd is still in use.

Fixes #622

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