net: IP defrag: encapsulate rbtree defrag code into callable functions

net: IP defrag: encapsulate rbtree defrag code into callable functions

BugLink: https://bugs.launchpad.net/bugs/1838349
[ Upstream commit c23f35d19db3b36ffb9e04b08f1d91565d15f84f ]

This is a refactoring patch: without changing runtime behavior,
it moves rbtree-related code from IPv4-specific files/functions
into .h/.c defrag files shared with IPv6 defragmentation code.

v2: make handling of overlapping packets match upstream.

Signed-off-by: Peter Oskolkov <posk@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>