6lowpan: use rbtree for IP frag queue

This patch aligns IP defragmenation logic in 6lowpan with that
of IPv4 and IPv6: see
commit d4289fcc

 ("net: IP6 defrag: use rbtrees for IPv6 defrag")

Modifying ip_defrag selftest seemed like an overkill, as I suspect
most kernel test setups do not have 6lowpan hwsim enabled. So I ran
the following code/script manually:

	insmod ./mac802154_hwsim.ko

	iwpan dev wpan0 set pan_id 0xbeef
	ip link add link wpan0 name lowpan0 type lowpan
	ip link set wpan0 up
	ip link set lowpan0 up

	iwpan dev wpan1 set pan_id 0xbeef
	ip netns add foo
	iwpan phy1 set netns name foo
	ip netns exec foo ip link add link wpan1 name lowpan1 type lowpan
	ip netns exec foo ip link set wpan1 up
	ip netns exec foo ip link set lowpan1 up

	ip -6 addr add "fb01::1/128" nodad dev lowpan0
	ip -netns foo -6 addr add "fb02::1/128" nodad dev lowpan1

	ip -6 route add "fb02::1/128" dev lowpan0
	ip -netns foo -6 route add "fb01::1/128" dev lowpan1

	# then in term1:
	   ip netns exec foo bash
	   ./udp_stream -6

	# in term2:
	    ./udp_stream -c -6 -H fb02::1

	# pr_warn_once showed that the code changed by this patch
	# was invoked.

Signed-off-by: Peter Oskolkov <posk@google.com>
Acked-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>