ifnet: Add some sanity checks

To be more robust since the checking is now performed where the
interface is referenced.

While here, remove a redundant check from if_vmove_loan().

Reviewed by: kp, glebius, pouria
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55875

(cherry picked from commit 00d96da231d007673a1672452748d8ea4f6788ae)