Allow kernel to compile without BPF.

r297816 added some bpf magic for VIMAGE unconditionally which no longer
allows kernels to compile without bpf (but with other networking).
Add the missing ifdef checks and allow a kernel to compile without bpf
again.

PR:		[[ https://bugs.freebsd.org/242136 | 242136 ]]
Reported by:	dave mischler.com
MFC after:	2 weeks