Add a comment explaining that we initialize the 'a' buffer for

zero-copy to the store buffer position on the BPF descriptor,
and the 'b' buffer as the free buffer in order to fill them in
the order documented in bpf(4).

MFC after:	4 months
Suggested by:	csjp

sys/net/bpf_zerocopy.c

@@ -496,6 +496,11 @@ bpf_zerocopy_ioctl_setzbuf(struct thread *td, struct bpf_d *d,
 		zbuf_free(zbb);
 		return (EINVAL);
 	}
+
+	/*
+	 * Point BPF descriptor at buffers; initialize sbuf as zba so that
+	 * it is always filled first in the sequence, per bpf(4).
+	 */
 	d->bd_fbuf = (caddr_t)zbb;
 	d->bd_sbuf = (caddr_t)zba;
 	d->bd_slen = 0;