nlecomposition: Ensure flushes after seek have the right seqnum
Seeks that lead to a stack change lead to deactivating the current stack. At that point we explicitely flush downstream as a reaction to the flushing seek. Until now those flushes had a random seqnum, this fails if we are a nested compostion as the parent composition will end up dropping that flush which in turns might lead to deadlocks. For example, the flush goes through a `compositor` which wants to flush downstream to stop its srcpad task, but that flush wouldn't have "released" its srcpad thread if the composition srcpad drops it, meaning it won't be able to stop the task ever.
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