Fix: add missing RCU read unlock

Master commit 24eb8569dc841f318d7964ca9a0ad0a4f9508a8e
"Fix: Missing RCU read locks in syscall_list_channel()"

Introduces unbalanced RCU read-side lock/unlock. Should be applied to
master and stable-2.6.

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