thp: clear compound mapping

Clear compound mapping for anonymous compound pages like it already
happens for regular anonymous pages.  But crash if mapping is set for any
tail page, also the PageAnon check is meaningless for tail pages.  This
check only makes sense for the head page, for tail page it can only hide
bugs and we definitely don't want to hide bugs.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>