ifnet: Use NET_EPOCH_WAIT() macro

This makes it easier to grep the usage.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45715

(cherry picked from commit 71f8fbf9bda43e3ca2f26d0499b7a88d0f2f7352)
(cherry picked from commit 338bb091008f32fda5cdd8cf9484fa942fa0937c)