Fix rounding method used to compute huge pages

Hi,

When computing the dynamic value of shared_memory_size_in_huge_pages,
(1+size_b/hp_size) is currently used. This works when size_b is not
divisible by hp_size. However, it will yield an additional huge page
when size_b is divisible by hp_size.

On CreateAnonymousSegment's side, the allocation size is rounded up to
the next required huge pages when necessary. However, there's no
overflow check when doing this round up.

0001: This patch replicates CreateAnonymousSegment's rounding method
to InitializeShmemGUCs, only rounding up when the value is not
divisible by hp_size.

0002: This patch uses add_size in CreateAnonymousSegment when the
allocation size is rounded up, to check for possible overflow.

Regards,
Anthonin Bonnefoy