linuxkpi: Correct nents passed to dma_{un,}map_sg_attrs

[zishun.yi.dev_gmail.com]

Summary

In linux docs, there is:

void
dma_unmap_sg(struct device *dev, struct scatterlist *sg,
             int nents, enum dma_data_direction direction)
Note: <nents> must be the number you passed in, not the number of DMA
address entries returned.

but I think currently nents is equal to orig_nents in linuxkpi. So It's
just a trivial fix.

[aokblast]

LGTM!

[bz]

I believe this is a NOP but more correct.