pgsql: Fix some rounding code for shared memory.

Fix some rounding code for shared memory.

InitializeShmemGUCs() always added 1 to the value calculated for
shared_memory_size_in_huge_pages, which is unnecessary if the
shared memory size is divisible by the huge page size.

CreateAnonymousSegment() neglected to check for overflow when
rounding up to a multiple of the huge page size.

These are arguably bugs, but they seem extremely unlikely to be
causing problems in practice, so no back-patch.

Author: Anthonin Bonnefoy <anthonin(dot)bonnefoy(at)datadoghq(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAO6_Xqq2vZbva0R9eQSY0p2kfksX2aP4r%3D%2BZ_q1HBYNU%3Dm8bBg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8eef2df1898cc34dfaa69ff200f5112d7eeb7c67

Modified Files
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src/backend/port/sysv_shmem.c | 2 +-
src/backend/storage/ipc/ipci.c | 4 +++-
2 files changed, 4 insertions(+), 2 deletions(-)