Fix in-place shrinking huge reallocation purging bugs.

Fix the shrinking case of huge_ralloc_no_move_similar() to purge the
correct number of pages, at the correct offset.  This regression was
introduced by 8d6a3e83 (Implement
dynamic per arena control over dirty page purging.).

Fix huge_ralloc_no_move_shrink() to purge the correct number of pages.
This bug was introduced by 96739834
(Purge/zero sub-chunk huge allocations as necessary.).