ifnet: allocate index at the end of if_alloc_domain()

Now that if_alloc_domain() never fails and actually doesn't
expose ifnet to outside we can eliminate IFNET_HOLD and two
step index allocation.

Reviewed by: kp
Differential revision: https://reviews.freebsd.org/D33259

(cherry picked from commit 8062e5759cb4886ad4630d52c212d8ca77ef9c95)