From: William Lee Irwin III <wli@holomorphy.com>

follow_hugetlb_page() drops out of the loop prematurely and fails to take the
appropriate refcounts if its starting address was not hugepage-aligned.

It looked a bit unclean too, so I rewrote it.  This fixes a bug, and more
importantly, makes the thing readable by something other than a compiler
(e.g.  programmers).