BIOCSETBUFMODE ioctl call on bpf with BPF_BUFMODE_ZBUF argument always returns EBUSY.
Reason: 
bpfopen() in sys/net/bpf.c calls bpf_buffer_ioctl_sblen() on every opened bpf device. 
bpf_buffer_ioctl_sblen() initializes bd_fbuf and bd_sbuf to freshly allocated memory buffers. 
Therefore later in BIOCSETBUFMODE ioctl the following condition is always true:
                if (d->bd_sbuf != NULL || d->bd_hbuf != NULL ||
                    d->bd_fbuf != NULL || d->bd_bif != NULL) {
                        BPFD_UNLOCK(d);
                        CURVNET_RESTORE();
                        return (EBUSY);
                }

Solution: 
Insert this code in BIOCSETBUFMODE ioctl:
                if(*(u_int *)addr == BPF_BUFMODE_ZBUF && d->bd_bufmode == BPF_BUFMODE_BUFFER) {
                         bpf_buffer_free(d);
                        d->bd_sbuf = 0;
                        d->bd_fbuf = 0;
                        d->bd_hbuf = 0;
                }

This works for switching to zerocopy mode. Probably some checking and action is needed if someone tries to switch back to BPF_BUFMODE_BUFFER mode after zerocopy mode.

10.1-RELEASE has the same problem.