generic, x86: fix add iommu_num_pages helper function

This IOMMU helper function doesn't work for some architectures:

  http://marc.info/?l=linux-kernel&m=121699304403202&w=2

It also breaks POWER and SPARC builds:

  http://marc.info/?l=linux-kernel&m=121730388001890&w=2

Currently, only x86 IOMMUs use this so let's move it to x86 for
now.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/pci-dma.c

@@ -123,6 +123,14 @@ void __init pci_iommu_alloc(void)
 
 	pci_swiotlb_init();
 }
+
+unsigned long iommu_num_pages(unsigned long addr, unsigned long len)
+{
+	unsigned long size = roundup((addr & ~PAGE_MASK) + len, PAGE_SIZE);
+
+	return size >> PAGE_SHIFT;
+}
+EXPORT_SYMBOL(iommu_num_pages);
 #endif
 
 /*