nbtree: Allocate new pages in separate function.

Split nbtree's _bt_getbuf function is two: code that read locks or write
locks existing pages remains in _bt_getbuf, while code that deals with
allocating new pages is moved to a new, dedicated function called
_bt_allocbuf.  This simplifies most _bt_getbuf callers, since it is no
longer necessary for them to pass a heaprel argument.  Many of the
changes to nbtree from commit 61b313e4 can be reverted.  This minimizes
the divergence between HEAD/PostgreSQL 16 and earlier release branches.

_bt_allocbuf replaces the previous nbtree idiom of passing P_NEW to
_bt_getbuf.  There are only 3 affected call sites, all of which continue
to pass a heaprel for recovery conflict purposes.  Note that nbtree's
use of P_NEW was superficial; nbtree never actually relied on the P_NEW
code paths in bufmgr.c, so this change is strictly mechanical.

GiST already took the same approach; it has a dedicated function for
allocating new pages called gistNewBuffer().  That factor allowed commit
61b313e4 to make much more targeted changes to GiST.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CAH2-Wz=8Z9qY58bjm_7TAHgtW6RzZ5Ke62q5emdCEy9BAzwhmg@mail.gmail.com

src/backend/access/heap/heapam_handler.c

@@ -731,14 +731,9 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 								 *multi_cutoff);
 
 
-	/*
-	 * Set up sorting if wanted. NewHeap is being passed to
-	 * tuplesort_begin_cluster(), it could have been OldHeap too. It does not
-	 * really matter, as the goal is to have a heap relation being passed to
-	 * _bt_log_reuse_page() (which should not be called from this code path).
-	 */
+	/* Set up sorting if wanted */
 	if (use_sort)
-		tuplesort = tuplesort_begin_cluster(oldTupDesc, OldIndex, NewHeap,
+		tuplesort = tuplesort_begin_cluster(oldTupDesc, OldIndex,
 											maintenance_work_mem,
 											NULL, TUPLESORT_NONE);
 	else