mm, hugetlbfs: introduce ->pagesize() to vm_operations_struct

mm, hugetlbfs: introduce ->pagesize() to vm_operations_struct

When device-dax is operating in huge-page mode we want it to behave like
hugetlbfs and report the MMU page mapping size that is being enforced by
the vma.

Similar to commit 31383c6865a5 "mm, hugetlbfs: introduce ->split() to
vm_operations_struct" it would be messy to teach vma_mmu_pagesize()
about device-dax page mapping sizes in the same (hstate) way that
hugetlbfs communicates this attribute.  Instead, these patches introduce
a new ->pagesize() vm operation.

Link: http://lkml.kernel.org/r/151996254734.27922.15813097401404359642.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]>