Small patch to multicast code...

[Bruce M. Simpson]

gnn at FreeBSD.org wrote:
> Somehow the data that the device needs to do the proper checksum
> offload is getting trashed here.  Now, since it's clear we need a
> writable packet structure so that we don't trash the original, I'm
> wondering if the m_pullup() will be sufficient.
>   

If it's serious enough to break UDP checksumming on the wire, perhaps we 
should just swallow the mbuf allocator heap churn and do the m_dup() for 
now, but slap in a big comment about why it's there.

BMS