Fix shared memory size calculation on 64-bit systems (#520)

Fix shared memory size calculation on 64-bit systems (#520)

Since commit 2253ee0, the wrong type (uint32 instead of size_t) was used
to calculate the PagePool::theLevels size. theLevels memory (positioned
by different and correct code) did not overlap with the raw pages
buffer, but the raw pages buffer could, in some cases, be 32 bits short,
placing the last 4 bytes of the last page outside of allocated memory.

In practice, shared memory allocations are page-aligned, and the
difference in 4 bytes was probably compensated by the extra allocated
bytes in most (or perhaps even all) cases.