Inconsistency in vmem_mix_allocations test

[PietrasMaciej]

In vmem_mix_allocations test: https://github.com/pmem/pmdk/blob/master/src/test/vmem_mix_allocations/vmem_mix_allocations.c
there is a mistake or defect. At 78 line comment says:

/* test with multiple size of allocations from 4MB to 2B */
for (i = 0; i < COUNT; ++i, obj_size /= 2) {

but the allocation starts from 8MB, not 4MB, cause obj_size's value is 4<<21. Therefore the test should be examined if is doing exactly what should do or just comment should be changed.

[PietrasMaciej]

fixed