Fix: sessiond: ust-app: account for the event notification pipes fds

Fix: sessiond: ust-app: account for the event notification pipes fds

The file descriptors of the event notifier group's event pipe are not
accounted by the lttng_fd_*() module, which tracks the use the file
descriptors by the userspace tracer.

Add the necessary lttng_fd_*() uses to account for the pipe's file
descriptors over their lifetime.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib24b4c2934655573965f06fa7998859335053083