MFC r345292:

MFC r345292:
  Convert allocation of bpf_if in bpfattach2 from M_NOWAIT to M_WAITOK
  and remove possible panic condition.

  It is already allowed to sleep in bpfattach[2], since BPF_LOCK was
  converted to SX lock in r332388. Also move KASSERT() to the top of
  function and make full initialization before bpf_if will be linked
  to BPF's list of interfaces.