Change the semantics to retire any buffer older than the current seqno
rather than repeatedly calling calling the function to retire the
buffer at the head of the list matching the request seqno.

Whilst this should have no semantic impact on the implementation, Daniel
was wondering if there was a bug where we might miss a retirement and so
end up with a continually growing active list.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>