btrfs: optimize balance_level() path reference handling

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>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>