dma: prevent races from sharing fd between children

dma: prevent races from sharing fd between children

On fork, fds are shared between children.  If two processes work on
different recipients, but on the same queue file, they might get
confused when the fd (and thus the offset) is shared.  Prevent this by
re-opening the queue file after fork.

Reported-by: Daniel Roethlisberger <daniel@roe.ch>