Fix: fd leak when closing metadata stream

Make sure to close the metadata poll pipe when the endpoint of the
stream dies out (ex: relayd stops unexpectedly). That was causing a fd
leak when we were destroying a metadata stream without using the close
metadata code path.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: in lttng_read/write deny count bigger than the possible returned value

Signed-off-by: David Goulet <dgoulet@efficios.com>