mem: Fix bug in PhysicalMemory use of mmap and munmap

mem: Fix bug in PhysicalMemory use of mmap and munmap

This patch fixes a bug in how physical memory used to be mapped and
unmapped. Previously we unmapped and re-mapped if restoring from a
checkpoint. However, we never checked that the new mapping was
actually the same, it was just magically working as the OS seems to
fairly reliably give us the same chunk back. This patch fixes this
issue by relying entirely on the mmap call in the constructor.