PG16-PG17: call RelationGetSmgr instead of custom code

[ivan-v-kush]

It's a preparation PR for PG16 to migrate hydra to PG17. Can be merged in PG16 version

This code should be changed in PG17, will be added one more #elif.

	if (unlikely(rel->rd_smgr == NULL))
	BlockNumber nblocks = smgrnblocks(RelationGetSmgr(rel), MAIN_FORKNUM);
	{
	#if PG_VERSION_NUM >= PG_VERSION_16
		smgrsetowner(&(rel->rd_smgr), smgropen(rel->rd_locator, rel->rd_backend));
	#else
		smgrsetowner(&(rel->rd_smgr), smgropen(rel->rd_node, rel->rd_backend));
	#endif
	}

Will be changedto smgrpin See MR in Postgres

Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Wed Jan 31 12:31:02 2024 +0200
Discussion: https://www.postgresql.org/message-id/CA%2BhUKGJ8NTvqLHz6dqbQnt2c8XCki4r2QvXjBQcXpVwxTY_pvA@mail.gmail.com

But this logic is included in the standard function RelationGetSmgr. Also see the same code in the Citus repository

https://github.com/citusdata/citus/blob/f7bead22d478ac3f407b1fb0f23739a289743bcc/src/backend/columnar/columnar_tableam.c#L1891