frag6: move public structure into file local space.

Move ip6asfrag and the accompanying IP6_REASS_MBUF macro from
ip6_var.h into frag6.c as they are not used outside frag6.c.
Sadly struct ip6q is all over the mac framework so we have to
leave it public.

This reduces the public KPI space.

MFC after: 3 months
X-MFC: possibly MFC the #define only to stable branches
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