This change works towards bringing down allocations for manager.Stat().
There's two things this change does:

1. Swap to opening a file and issuing a single read for uint64/"max" values.
Previously we were doing os.ReadFile's which returns a byte slice, so it needs
to allocate.
2. Sizing the map we store {controller}.stat values in. We store 40+ stats in this
map and the default bucket size for Go seems to be smaller than this, so we'd incur
a couple allocs at runtime when adding these.

Signed-off-by: Danny Canter <danny@dcantah.dev>

In the journey of continuing to reduce allocations in manager.Stat,
this sets its eyes on optimizing readHugeTlbStats. The number of allocs
shaved off here is tied to the number of files in the cgroup directory
as f.Readdir(-1) was being called which returns a slice of interfaces.
To optimize this we can use a trick runc's cgroup code does which is to
take advantage of the fact that we know exactly what files we need to
open as they're fixed, the only variable portion is the hugepage sizes
available on the host. To get around this we can compute the hugepage
sizes in a sync.Once, and all future manager.Stat calls can reap the
benefits as we don't need to Readdir() the entire cgroup path to search
for hugetlb.pagesize.foobar's.

This brings the total number of allocations on Go 1.19.4 down to 199 from
the starting point of 322.

Signed-off-by: Danny Canter <danny@dcantah.dev>
Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com>