linux/lttng: Fix return type and names of layout's network events

linux/lttng: Fix return type and names of layout's network events

All new events added to the layout should return a Collection instead of a
single string. This patch updates the network event's signature

Change-Id: I978f760d6f72714467511f3c58c7a9f3a134830d
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/78070
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>