ifnet: Move SIOCSIFVNET from ifhwioctl() to ifioctl()

SIOCSIFVNET is not a hardware ioctl. Move it to where it belongs.

Where here, rewrite the logic of checking whether we are moving the
interface from and to the same vnet or not, since it is obviously not
stable to access the interface's vnet, given the current thread may
race with other threads those running if_vmove().

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55880