Refactors VFile::is_seekable to use VFile::vnode() instead

[SuchAFuriousDeath]

@ultimaweapon By the way, I think that the vnode item field should be removed and each corresponding variant should be inside VnodeType.

Currently, we do unnecesarry checks - FreeBSD has to check the vnode type to ensure that v_un is a pointer of the right type anyway.