Memory leak for EMSGSIZE errors when writing to a BPF device

[Guy Harris]

bpfwrite() does:

	error = bpf_movein(uio, (int)d->bd_bif->bif_dlt, &m, &dst, &datlen);
	if (error)
		return(error);
	if (datlen > ifp->if_mtu)
		return(EMSGSIZE);
The mbuf chain pointed to by m isn't freed in that case.

FreeBSD 5.x fixes this by passing the MTU as an argument to 
bpf_movein(), and has bpf_movein() return EMSGSIZE before copying 
anything if the data length is greater than the MTU.

Current CVS NetBSD and OpenBSD free the mbuf chain before returning 
EMSGSIZE.