pmm: simpler and bugfix process_memory_range loop

[sktt]

This simplifies the adding of frames during boot. It also fixes a bug for configurations with > 1GB memory. After the initial 4K frames, the current (cur) is aligned to 2MB. Trying to add a 1GB frame at this point will lead to page faults further down the line as we end up with misaligned page frame mfns.

I am pretty unsure about what I did to the index <= first_avail_region (completely dropped it). Let me know if this should be included still somehow.