net: ipv4: place control buffer handling away from fragmentation iterators

Deal with the IPCB() area away from the iterators.

The bridge codebase has its own control buffer layout, move specific
IP control buffer handling into the IPv4 codepath.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>