btrfs: optimize balance_level() path reference handling

Instead of incrementing refcount on left node when it's referenced by path,
simply transfer ownership to path and set left to NULL. This eliminates:

- Unnecessary refcount increment/decrement operations
- Redundant conditional checks for left node cleanup

The path now consistently owns the left node reference when used.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: SaltyKitkat

fs/btrfs/ctree.c

@@ -1127,11 +1127,12 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 	/* update the path */
 	if (left) {
 		if (btrfs_header_nritems(left) > orig_slot) {
-			refcount_inc(&left->refs);
 			/* left was locked after cow */
 			path->nodes[level] = left;
 			path->slots[level + 1] -= 1;
 			path->slots[level] = orig_slot;
+			/* left is now owned by path */
+			left = NULL;
 			if (mid) {
 				btrfs_tree_unlock(mid);
 				free_extent_buffer(mid);
@@ -1151,8 +1152,7 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 		free_extent_buffer(right);
 	}
 	if (left) {
-		if (path->nodes[level] != left)
-			btrfs_tree_unlock(left);
+		btrfs_tree_unlock(left);
 		free_extent_buffer(left);
 	}
 	return ret;