Fix: kretprobe: null ptr deref on session destroy

The `filter_bytecode_runtime_head` list is currently not initialized for
the return event of the kretprobe. This caused a kernel null ptr
dereference when destroying a session. It can reproduced with the
following commands:

  lttng create
  lttng enable-event -k --function=lttng_test_filter_event_write my_event
  lttng start
  lttng stop
  lttng destroy

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I1162ce8b10dd7237a26331531f048346b984eee7