mm: fix false-positive WARN_ON() in truncate/invalidate for hugetlb

Hugetlb pages have ->index in size of the huge pages (PMD_SIZE or
PUD_SIZE), not in PAGE_SIZE as other types of pages.  This means we
cannot user page_to_pgoff() to check whether we've got the right page
for the radix-tree index.

Let's introduce page_to_index() which would return radix-tree index for
given page.

We will be able to get rid of this once hugetlb will be switched to
multi-order entries.

Fixes: fc127da0 ("truncate: handle file thp")
Link: http://lkml.kernel.org/r/20161123093053.mjbnvn5zwxw5e6lk@black.fi.intel.com


Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Doug Nelson <doug.nelson@intel.com>
Tested-by: Doug Nelson <doug.nelson@intel.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>