Get upper or lower dentry we should be using
struct dentry * d_backing_dentry(struct dentry * upper);
upper
The upper layer
This is the helper that should be used to get the dentry of the inode that will be used if this dentry were opened as a file. It may be the upper dentry or it may be a lower dentry pinned by the upper.
Normal filesystems should not use this to access their own dentries.