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 preempted 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.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib59b634bbaefd2444751547d20a891c9dd93cd73