MFC ec52ff6d1411 and 747feea146d8:
Streamline the infiniband code according to the ethernet code.

Specifically implement the if_requestencap callback function for infiniband.
Most of the changes are simply a cut and paste of the equivalent ethernet part.

Reviewed by: melifaro @
Differential Revision: https://reviews.freebsd.org/D27631
Sponsored by: Mellanox Technologies // NVIDIA Networking