PMEMAllocator does not correctly populate PMEM space

[tim gretler]

Populate space sometimes fails to initialize the pmem space. In PMEMAllocator::PopulateSpace it is possible that pmem_memset gets called on memory that is not part of the file. It happens if pu (cpu count) is not a power of 2. In my case pu was 12 and I used linux memmap. This caused a segmentation fault.

Reproduce:

• Setup linux memmap
• Run tutorial
• Check that pu is 12

Solutions:

• Always set pu to 16 as done here and check file is the power of 2
• Handle edge case for cases where cpu count not power of two

[Jiayu Wu]

Thanks for submitting, problem resolved #51