nfp: make use of the DMA_ATTR_SKIP_CPU_SYNC attr

DMA unmap may destroy changes CPU made to the buffer.  To make XDP
run correctly on non-x86 platforms we should use the
DMA_ATTR_SKIP_CPU_SYNC attribute.

Thanks to using the attribute we can now push the sync operation to the
common code path from XDP handler.

A little bit of variable name reshuffling is required to bring the
code back to readable state.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>