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.