Fix shmem allocation of fixed-sized custom stats kind

Fix shmem allocation of fixed-sized custom stats kind

StatsShmemSize(), that computes the shmem size needed for pgstats,
includes the amount of shared memory wanted by all the custom stats
kinds registered.  However, the shared memory allocation was done by
ShmemAlloc() in StatsShmemInit(), meaning that the space reserved was
not used, wasting some memory.

These extra allocations would show up under "<anonymous>" in
pg_shmem_allocations, as the allocations done by ShmemAlloc() are not
tracked by ShmemIndexEnt.

Issue introduced by 7949d9594582.

Author: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/04b04387-92f5-476c-90b0-4064e71c5f37@iki.fi
Backpatch-through: 18