The calculation of Maxmem was skipping the last phys_avail segment,
because of a wrong stop condition.
This was detected when using QEMU/PowerNV with Radix MMU and
low memory (2G). In this case opal_pci would allocate a DMA window
that was too small to cover all physical memory, resulting in reading all
zeroes from disk when using memory that was not inside the allocated
window.