infiniband: Convert BPF handling for IfAPI

Summary:
All callers of infiniband_bpf_mtap() call it through the wrapper macro,
which checks the if_bpf member explicitly. Since this is getting
hidden, move this check into the internal function and remove the
wrapper macro.

Reviewed by: hselasky
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39024