Fix checkpointer shared memory allocation

Fix checkpointer shared memory allocation

Use Min(NBuffers, MAX_CHECKPOINT_REQUESTS) instead of NBuffers in
CheckpointerShmemSize() to match the actual array size limit set in
CheckpointerShmemInit().  This prevents wasting shared memory when
NBuffers > MAX_CHECKPOINT_REQUESTS.  Also, fix the comment.

Reported-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/1439188.1754506714%40sss.pgh.pa.us
Author: Xuneng Zhou <[email protected]>
Co-authored-by: Alexander Korotkov <[email protected]>