bufmgr: Fix use of wrong variable in GetPrivateRefCountEntrySlow()

bufmgr: Fix use of wrong variable in GetPrivateRefCountEntrySlow()

Unfortunately, in 30df61990c67, I made GetPrivateRefCountEntrySlow() set a
wrong cache hint when moving entries from the hash table to the faster array.
There are no correctness concerns due to this, just an unnecessary loss of
performance.

Noticed while testing the index prefetching patch.

Discussion: https://postgr.es/m/CAH2-Wz=g=JTSyDB4UtB5su2ZcvsS7VbP+ZMvvaG6ABoCb+s8Lw@mail.gmail.com