Refactor ip_fragment() and ip6_fragment(). Use a mbuf list to

Refactor ip_fragment() and ip6_fragment().  Use a mbuf list to
simplify the handling of the fragment list.  Now the functions
ip_fragment() and ip6_fragment() always consume the mbuf.  They
free the mbuf and mbuf list in case of an error and take care about
the counter.  Adjust the code a bit to make v4 and v6 look similar.
Fixes a potential mbuf leak when pf_route6() called pf_refragment6()
and it failed.  Now the mbuf is always freed by ip6_fragment().
OK dlg@ mvs@