Use smgrexists() instead of access() to enforce uniqueness of generated relfilenumber (#7992)

## Problem

Postgres is using `access()` function in `GetNewRelFileNumber` to check
if assigned relfilenumber is not used for any other relation. This check
will not work in Neon, because we do not have all files in local
storage.

## Summary of changes

Use smgrexists() instead which will check at page server if such
relfilenode is used.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon_test_utils/neon_test_utils--1.3.sql

@@ -7,6 +7,12 @@ AS 'MODULE_PATHNAME', 'test_consume_xids'
 LANGUAGE C STRICT
 PARALLEL UNSAFE;
 
+CREATE FUNCTION test_consume_oids(oid int)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'test_consume_oids'
+LANGUAGE C STRICT
+PARALLEL UNSAFE;
+
 CREATE FUNCTION test_consume_cpu(seconds int)
 RETURNS VOID
 AS 'MODULE_PATHNAME', 'test_consume_cpu'

pgxn/neon_test_utils/neontest.c

@@ -35,6 +35,7 @@ PG_MODULE_MAGIC;
 extern void _PG_init(void);
 
 PG_FUNCTION_INFO_V1(test_consume_xids);
+PG_FUNCTION_INFO_V1(test_consume_oids);
 PG_FUNCTION_INFO_V1(test_consume_cpu);
 PG_FUNCTION_INFO_V1(test_consume_memory);
 PG_FUNCTION_INFO_V1(test_release_memory);
@@ -74,6 +75,21 @@ _PG_init(void)
 
 #define neon_read_at_lsn neon_read_at_lsn_ptr
 
+/*
+ * test_consume_oids(int4), for rapidly consuming OIDs, to test wraparound.
+ * Unlike test_consume_xids which is passed number of xids to be consumed,
+ * this function is given the target Oid.
+ */
+Datum
+test_consume_oids(PG_FUNCTION_ARGS)
+{
+	int32 oid = PG_GETARG_INT32(0);
+
+	while (oid != GetNewObjectId());
+
+	PG_RETURN_VOID();
+}
+
 /*
  * test_consume_xids(int4), for rapidly consuming XIDs, to test wraparound.
  */