infiniband: Opt-in for net epoch

This is counterpart to e87c4940156c, which did the same for ethernet.

Suggested by: hselasky
Reviewed by: hselasky, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39405

(cherry picked from commit fc6c93b6a5cf14cbc099f550f745d63779499fec)