iflib: Use if_alloc_dev() to allocate the ifnet

This ensures that the ifnet's NUMA affinity is accurate.

Reviewed by: kbowling
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46667

(cherry picked from commit 767723ddebe9c76a2d4a45a50d9b0efc9f2f91d7)