device-dax: implement ->pagesize() for smaps to report MMUPageSize

Given that device-dax is making similar page mapping size guarantees as
hugetlbfs, emit the size in smaps and any other kernel path that
requests the mapping size of a vma.

Link: http://lkml.kernel.org/r/151996255287.27922.18397777516059080245.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <[email protected]>
Reported-by: Jane Chu <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Paul Mackerras <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
(cherry picked from commit c1d53b9)

Orabug: 27262713

Signed-off-by: Jane Chu <[email protected]>
Reviewed-by: Steve Sistare <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>

Author: djbw

drivers/dax/device.c

@@ -439,10 +439,20 @@ static int dev_dax_split(struct vm_area_struct *vma, unsigned long addr)
 	return 0;
 }
 
+static unsigned long dev_dax_pagesize(struct vm_area_struct *vma)
+{
+	struct file *filp = vma->vm_file;
+	struct dev_dax *dev_dax = filp->private_data;
+	struct dax_region *dax_region = dev_dax->region;
+
+	return dax_region->align;
+}
+
 static const struct vm_operations_struct dax_vm_ops = {
 	.fault = dev_dax_fault,
 	.huge_fault = dev_dax_huge_fault,
 	.split = dev_dax_split,
+	.pagesize = dev_dax_pagesize,
 };
 
 static int dax_mmap(struct file *filp, struct vm_area_struct *vma)