Fix refreshing physical relfilenumber on shared index

Buildfarm member 'prion', which is configured with
-DRELCACHE_FORCE_RELEASE -DCATCACHE_FORCE_RELEASE, failed with errors
like this:

    ERROR:  could not read blocks 0..0 in file "global/2672": read only 0 of 8192 bytes

while running a parallel test group that includes VACUUM FULL on some
catalog tables among other things. I was not able to reproduce that
just by running the tests with -DRELCACHE_FORCE_RELEASE
-DCATCACHE_FORCE_RELEASE, even though 'prion' hit it on first run
after commit 2b9b8ebbf8, so there might be something else that makes
it more susceptible to the race. However, I was able to reproduce it
by adding another test to the same test group that runs "vacuum full
pg_database" repeatedly.

The problem is that RelationReloadIndexInfo() no longer calls
RelationInitPhysicalAddr() on a nailed, shared index, when an
invalidation happens early during backend startup, before the critical
relcaches have been built. Before commit 2b9b8ebbf8, that was done by
RelationReloadNailed(), but it went missing from that path. Add it
back as an explicit step.

Broken by commit 2b9b8ebbf8, which refactored these functions.

Discussion: https://www.postgresql.org/message-id/db876575-8f5b-4193-a538-df7e1f92d47a%40iki.fi

src/backend/utils/cache/relcache.c

@@ -2253,11 +2253,14 @@ RelationReloadIndexInfo(Relation relation)
 	 * If it's a shared index, we might be called before backend startup has
 	 * finished selecting a database, in which case we have no way to read
 	 * pg_class yet.  However, a shared index can never have any significant
-	 * schema updates, so it's okay to ignore the invalidation signal.  Just
-	 * mark it valid and return without doing anything more.
+	 * schema updates, so it's okay to mostly ignore the invalidation signal.
+	 * Its physical relfilenumber might've changed, but that's all.  Update
+	 * the physical relfilenumber, mark it valid and return without doing
+	 * anything more.
 	 */
 	if (relation->rd_rel->relisshared && !criticalRelcachesBuilt)
 	{
+		RelationInitPhysicalAddr(relation);
 		relation->rd_isvalid = true;
 		return;
 	}