Fix oversized memory allocation in Parallel Hash Join
During the calculations of the maximum for the number of buckets, take into
account that later we round that to the next power of 2.
Reported-by: Karen Talarico
Bug: #16925
Discussion: https://postgr.es/m/16925-
ec96d83529d0d629%40postgresql.org
Author: Thomas Munro, Andrei Lepikhov, Alexander Korotkov
Reviewed-by: Alena Rybakina
Backpatch-through: 12