d_obtain_root — find or allocate a dentry for a given inode
|struct inode * inode|
Obtain an IS_ROOT dentry for the root of a filesystem.
We must ensure that directory inodes only ever have one dentry. If a dentry is found, that is returned instead of allocating a new one.
On successful return, the reference to the inode has been transferred
to the dentry. In case of an error the reference on the inode is
NULL or IS_ERR inode may be passed in and will be the
error will be propagate to the return value, with a
replaced by ERR_PTR(-ESTALE).