Add a configurable Descriptor Skip Length for systems that lack cache
coherence.

(akpm: I think this should be done as a module parameter, not a
compile-tinme option)
Signed-off-by: NRisto Suominen <Risto.Suominen@gmail.com>
Cc: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>