xen-blkfront: improve aproximation of required grants per request

Improve the calculation of required grants to process a request by
using nr_phys_segments instead of always assuming a request is going
to use all posible segments.

nr_phys_segments contains the number of scatter-gather DMA addr+len
pairs, which is basically what we put at every granted page.
for_each_sg iterates over the DMA addr+len pairs and uses a grant
page for each of them.

Signed-off-by: Roger Pau Monné <[email protected]>
Reviewed-by: David Vrabel <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: David Vrabel <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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