smgr: Make SMgrRelation initialization safer against errors

In case the smgr_open callback failed, the ->pincount field would not be
initialized and the relation would not be put onto the unpinned_relns list.

This buglet was introduced in 21d9c3ee4e, in 17.

Discussion: https://postgr.es/m/3vae7l5ozvqtxmd7rr7zaeq3qkuipz365u3rtim5t5wdkr6f4g@vkgf2fogjirl
Backpatch-through: 17

src/backend/storage/smgr/smgr.c

@@ -234,12 +234,12 @@ smgropen(RelFileLocator rlocator, ProcNumber backend)
 			reln->smgr_cached_nblocks[i] = InvalidBlockNumber;
 		reln->smgr_which = 0;	/* we only have md.c at present */
 
-		/* implementation-specific initialization */
-		smgrsw[reln->smgr_which].smgr_open(reln);
-
 		/* it is not pinned yet */
 		reln->pincount = 0;
 		dlist_push_tail(&unpinned_relns, &reln->node);
+
+		/* implementation-specific initialization */
+		smgrsw[reln->smgr_which].smgr_open(reln);
 	}
 
 	return reln;