Fix invalid madvise() during concurrent alloc/dealloc of MemoryDealer

Currently, madvise(MADV_REMOVE) is called after deallocation.
Another thread might allocate (and even write) the same region between
deallocation and madvise(), in which case the new thread will fail to read
what it have written.  So, call deallocate() after madvise(MADV_REMOVE).

Bug: 5654596
Change-Id: I26f36cd6013de499090768a0ddc68206a4a68219