Fix memory leaks in mmap/munmap

mmap/munmap and its cousins mmu::map_*() used to leak the 48-byte "vma"
object, twice. The idea to separate reserve() and map() was a good one,
but it was implemented incorrectly causing a vma object to be allocated
twice per mmap, and both of them were leaked when evacuate() didn't free
them.

So switched to a simpler implementation, where the "vma" object is internal
to mmu.cc, and not used by any of the callers; The struct vma is now properly
allocated only once per mmap(), and freed on munmap().