sysclk-lwla: Streamline packet output logic.

sysclk-lwla: Streamline packet output logic.

(process_sample_data): When expanding run-length samples into
session packets, calculate the number of samples to write in
advance while honoring all constraints.  This is cleaner than
checking constraints within the expansion loop.  Also, the new
logic always fills up packets exactly to whatever limit applies
first, thereby removing the need for truncation after the fact.