Remove direct call to GetNewObjectId() for pg_auth_members.oid

This routine should not be called directly as mentioned at its top, so
replace it by GetNewOidWithIndex().  Issue introduced by 6566133 when
pg_auth_members.oid got added, so no backpatch is needed.

Author: Maciek Sakrejda
Discussion: https://postgr.es/m/CAOtHd0Ckbih7Ur7XeVyLAJ26VZOfTNcq9qV403bNF4uTGtAN+Q@mail.gmail.com

src/backend/commands/user.c

@@ -1850,7 +1850,8 @@ AddRoleMems(const char *rolename, Oid roleid,
 			}
 
 			/* get an OID for the new row and insert it */
-			objectId = GetNewObjectId();
+			objectId = GetNewOidWithIndex(pg_authmem_rel, AuthMemOidIndexId,
+										  Anum_pg_auth_members_oid);
 			new_record[Anum_pg_auth_members_oid - 1] = objectId;
 			tuple = heap_form_tuple(pg_authmem_dsc,
 									new_record, new_record_nulls);