mm/truncate: fix truncation for pages of arbitrary size

Remove the assumption that a compound page is HPAGE_PMD_SIZE, and the
assumption that any page is PAGE_SIZE.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SeongJae Park <sjpark@amazon.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Link: https://lkml.kernel.org/r/20200908195539.25896-10-willy@infradead.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>