Fix ordering issue with WAL operations in GIN fast insert path

Contrary to what is documented in src/backend/access/transam/README,
ginHeapTupleFastInsert() had a few ordering issues with the way it does
its WAL operations when inserting items in its fast path.

First, when using a separate list, XLogBeginInsert() was being always
called before START_CRIT_SECTION(), and in this case a second thing was
wrong when merging lists, as an exclusive lock was taken on the tail
page *before* calling XLogBeginInsert().  Finally, when inserting items
into a tail page, the order of XLogBeginInsert() and
START_CRIT_SECTION() was reversed.  This commit addresses all these
issues by moving the calls of XLogBeginInsert() after all the pages
logged are locked and pinned, within a critical section.

This has been applied first only on HEAD as of 56b6625, but as per
discussion with Tom Lane and Álvaro Herrera, a backpatch is preferred to
keep all the branches consistent and to respect the transam's README
where we can.

Author:  Matthias van de Meent, Zhang Mingli
Discussion: https://postgr.es/m/CAEze2WhL8uLMqynnnCu1LAPwxD5RKEo0nHV+eXGg_N6ELU88HQ@mail.gmail.com
Backpatch-through: 10

src/backend/access/gin/ginfast.c

@@ -285,9 +285,6 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 		memset(&sublist, 0, sizeof(GinMetaPageData));
 		makeSublist(index, collector->tuples, collector->ntuples, &sublist);
 
-		if (needWal)
-			XLogBeginInsert();
-
 		/*
 		 * metapage was unlocked, see above
 		 */
@@ -307,6 +304,9 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 
 			metadata->nPendingPages = sublist.nPendingPages;
 			metadata->nPendingHeapTuples = sublist.nPendingHeapTuples;
+
+			if (needWal)
+				XLogBeginInsert();
 		}
 		else
 		{
@@ -335,7 +335,10 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 			metadata->nPendingHeapTuples += sublist.nPendingHeapTuples;
 
 			if (needWal)
+			{
+				XLogBeginInsert();
 				XLogRegisterBuffer(1, buffer, REGBUF_STANDARD);
+			}
 		}
 	}
 	else
@@ -361,11 +364,11 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 
 		data.ntuples = collector->ntuples;
 
+		START_CRIT_SECTION();
+
 		if (needWal)
 			XLogBeginInsert();
 
-		START_CRIT_SECTION();
-
 		/*
 		 * Increase counter of heap tuples
 		 */