Move _bt_upgrademetapage() into critical section.

Any changes on page should be done in critical section, so move
_bt_upgrademetapage into critical section. Improve comment. Found by Amit
Kapila during post-commit review of 857f9c36.

Author: Amit Kapila

src/backend/access/nbtree/nbtinsert.c

@@ -2150,10 +2150,6 @@ _bt_newroot(Relation rel, Buffer lbuf, Buffer rbuf)
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
-	/* upgrade metapage if needed */
-	if (metad->btm_version < BTREE_VERSION)
-		_bt_upgrademetapage(metapg);
-
 	/*
 	 * Create downlink item for left page (old root).  Since this will be the
 	 * first item in a non-leaf page, it implicitly has minus-infinity key
@@ -2178,6 +2174,10 @@ _bt_newroot(Relation rel, Buffer lbuf, Buffer rbuf)
 	/* NO EREPORT(ERROR) from here till newroot op is logged */
 	START_CRIT_SECTION();
 
+	/* upgrade metapage if needed */
+	if (metad->btm_version < BTREE_VERSION)
+		_bt_upgrademetapage(metapg);
+
 	/* set btree special data */
 	rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 	rootopaque->btpo_prev = rootopaque->btpo_next = P_NONE;

src/backend/access/nbtree/nbtpage.c

@@ -341,10 +341,6 @@ _bt_getroot(Relation rel, int access)
 		LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
 		LockBuffer(metabuf, BT_WRITE);
 
-		/* upgrade metapage if needed */
-		if (metad->btm_version < BTREE_VERSION)
-			_bt_upgrademetapage(metapg);
-
 		/*
 		 * Race condition:	if someone else initialized the metadata between
 		 * the time we released the read lock and acquired the write lock, we
@@ -379,6 +375,10 @@ _bt_getroot(Relation rel, int access)
 		/* NO ELOG(ERROR) till meta is updated */
 		START_CRIT_SECTION();
 
+		/* upgrade metapage if needed */
+		if (metad->btm_version < BTREE_VERSION)
+			_bt_upgrademetapage(metapg);
+
 		metad->btm_root = rootblkno;
 		metad->btm_level = 0;
 		metad->btm_fastroot = rootblkno;

src/include/access/nbtree.h

@@ -103,7 +103,7 @@ typedef struct BTMetaPageData
 	BlockNumber btm_fastroot;	/* current "fast" root location */
 	uint32		btm_fastlevel;	/* tree level of the "fast" root page */
 	/* following fields are available since page version 3 */
-	TransactionId btm_oldest_btpo_xact; /* oldest btpo_xact among of deleted
+	TransactionId btm_oldest_btpo_xact; /* oldest btpo_xact among all deleted
 										 * pages */
 	float8		btm_last_cleanup_num_heap_tuples;	/* number of heap tuples
 													 * during last cleanup */