Fix: sample discarded events count before reserve

Sampling the discarded events count in the buffer_end callback is done
out of order, and may therefore include increments performed by following
events (in following packets) if the thread doing the end-of-packet
event write is interrupted for a long time.

Sampling the event discarded counts before reserving space for the last
event in a packet, and keeping this as part of the private ring buffer
context, should fix this race.

In lttng-modules, this scenario would only happen if an interrupt
handler produces many events, when nested over an event between its
reserve and commit. Note that if lttng-modules supports faultable
tracepoints in the future, this may become more easy to trigger due to
preemption.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I696a206b3926fc1abbee35caa9af65461ff56c68