dma(8): Fix race condition in multi-recipient delivery

[Matthew Dillon]

:Uh.  I ment to add a dup() there, but I moved a lot of code, so I 
:forgot...  What about the attached patch?
:
:cheers
:   simon

    dup() doesn't solve the problem.  A dup()'d descriptor still
    points to a common file pointer and thus a common seek position.

    Only open() will generate a new file pointer backing the file
    descriptor.

						-Matt