Internalizing M_EXT refcounting and lwbufs

[Samuel J. Greear]

I have had a patch kicking around for a while that I finally decided
to beat into committable form, that brings the reference count into
the m_ext structure. Currently there are very few users of M_EXT so I
just modified them all to manage the refcount manually with atomic
ops. Would we prefer this were functionalized, like FreeBSD has done?

Also, It would be great if we could stuff a lwbuf into m_ext, but the
only reasonable way I can see to do that is to add an additional
function pointer which would be used to get the stable reference, as
well as probably an offset, calling conventions would be slightly
different for free in this case, and that route just doesn't feel very
clean...  So I was wondering if anyone had any suggestions?

Thanks,
Sam