lib/vfscore: Initialize lists for stdio_vnode

lib/vfscore: Initialize lists for stdio_vnode

The VFS `struct vnode` structure has two `uk_list_head` members:
`v_link` and `v_names`. These need to be initialized by a dedicated
macro / function, such as `UK_LIST_HEAD_INIT`.

The `stdio_vnode` variable (of type `struct vnode`) is not initializing
these members (they are filled with zeros). This will cause crashes when
using them.

This commit fixes that by properly initializing the `v_link` and
`v_names` members of the `stdio_vnode` variable.

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>