Fix if_loop so bpfwrite() can use it regardless of the state of
bd_hdrcmplt. As if_loop does not use link-level headers, its behavior
when used by bpfwrite() should be the same regardless of the state of
bd_hdrcmplt. Without this change, libpcap (and other BPF users that
work like it) fail when writing to loopback interfaces.

Differential Revision: https://reviews.freebsd.org/D2989
Reviewed by: gnn, melifaro
Approved by: jmallett (mentor)
MFC after: 3 days