mt76: clear skb pointers from rx aggregation reorder buffer during cleanup

[ Upstream commit 9379df2fd9234e3b67a23101c2370c99f6af6d77 ]

During the cleanup of the aggregation session, a rx handler (or release timer)
on another CPU might still hold a pointer to the reorder buffer and could
attempt to release some packets.
Clearing pointers during cleanup avoids a theoretical use-after-free bug here.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>