filemap: update offset check in filemap_fault()

With THP, current check of offset:

VM_BUG_ON_PAGE(page->index != offset, page);

is no longer accurate. Update it to:

VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Song Liu <[e-mail]>
Acked-by: Rik van Riel <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Cc: Hillf Danton <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: William Kucharski <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/filemap.c

···
 		put_page(page);
 		goto retry_find;
 	}
-	VM_BUG_ON_PAGE(page->index != offset, page);
+	VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page);
 
 	/*
 	 * We have a locked page in the page cache, now we need to check