dax: use linear_page_index()

I was basically open-coding it (thanks to copying code from do_fault()
which probably also needs to be fixed).
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dax.c

@@ -529,7 +529,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	if ((pmd_addr + PMD_SIZE) > vma->vm_end)
 		return VM_FAULT_FALLBACK;
 
-	pgoff = ((pmd_addr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+	pgoff = linear_page_index(vma, pmd_addr);
 	size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	if (pgoff >= size)
 		return VM_FAULT_SIGBUS;