Fix: list_lttng_agent_events: unbalanced RCU read-side lock on error

The error label jumps to the end label which releases the RCU read-side
lock. There are many error paths in this function which goto error
without holding the RCU read-side lock, thus causing unbalanced RCU
read-side lock.

There is no point in keeping so short RCU read-side critical sections,
so cover the entire function with a single read-side critical section.

[ Applies to stable-2.12 and possibly prior versions. Does _not_ apply
  to stable-2.13+. ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5b20c229a5df22d22ecfdc64dbbb87ee118649d2