bcachefs: Optimize memory accesses in bch2_btree_node_get()

This puts a load behind some branches before where it's used, so that it
can execute in parallel with other loads.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_cache.c

@@ -775,16 +775,17 @@ struct btree *bch2_btree_node_get(struct btree_trans *trans, struct btree_path *
 
 	EBUG_ON(level >= BTREE_MAX_DEPTH);
 
-	if (c->opts.btree_node_mem_ptr_optimization) {
-		b = btree_node_mem_ptr(k);
-		/*
-		 * Check b->hash_val _before_ calling btree_node_lock() - this
-		 * might not be the node we want anymore, and trying to lock the
-		 * wrong node could cause an unneccessary transaction restart:
-		 */
-		if (b && b->hash_val == btree_ptr_hash_val(k))
+	b = btree_node_mem_ptr(k);
+
+	/*
+	 * Check b->hash_val _before_ calling btree_node_lock() - this might not
+	 * be the node we want anymore, and trying to lock the wrong node could
+	 * cause an unneccessary transaction restart:
+	 */
+	if (likely(c->opts.btree_node_mem_ptr_optimization &&
+		   b &&
+		   b->hash_val == btree_ptr_hash_val(k)))
 			goto lock_node;
-	}
 retry:
 	b = btree_cache_find(bc, k);
 	if (unlikely(!b)) {