The functionality of stat is split into smaller sub-functions, which allows
to reuse them more flexible, as the code flow is similar but not always the same.
By this, the ugly and incorrect re-setting of the i-node in the lookup callback
function is avoided.
The correct i-node is now calculated beforehand and stat simply creates a
`libc::stat` struct from the provided parameters.
Also, this fixes incorrect i-node assignments in the readdir callback function.
Signed-off-by: Christian Ebner <c.ebner@proxmox.com>