net: IP6 defrag: use rbtrees for IPv6 defrag

net: IP6 defrag: use rbtrees for IPv6 defrag

Currently, IPv6 defragmentation code drops non-last fragments that
are smaller than 1280 bytes: see
commit 0ed4229b08c1 ("ipv6: defrag: drop non-last frags smaller than min mtu")

This behavior is not specified in IPv6 RFCs and appears to break
compatibility with some IPv6 implemenations, as reported here:
https://www.spinics.net/lists/netdev/msg543846.html

This patch re-uses common IP defragmentation queueing and reassembly
code in IPv6, removing the 1280 byte restriction.

Signed-off-by: Peter Oskolkov <[email protected]>
Reported-by: Tom Herbert <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Florian Westphal <[email protected]>
Signed-off-by: David S. Miller <[email protected]>