Use vma_pages function on vma object instead of explicit computation.
Found by coccinelle spatch "api/vma_pages.cocci"
Reviewed-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NThomas Meyer <thomas@m3y3r.de>
Signed-off-by: NWill Deacon <will.deacon@arm.com>