HUGEPAGE could be larger on some platforms (e.g. 512M on aarch64 w/ 64K pages),
in which case it would cause grow_retained / exp_grow to over-reserve VMs.
Similarly, make sure the base alloc has a const 2M alignment.
This was promised in the review of the introduction of geom_grow, but would have
been painful to do there because of the series that introduced it. Now that
those are comitted, renaming is easier.
Qi Wang