net: ipv6: set pkt properties prior to routing

pkt properties should be set prior to routing.
If packet routing is turned on and the packet is
forwarded to an interface, the pkt properties like
ipv6_ext_len or ip_hdr_len will not be initialized.
If a UDP packet is forwarded to an 6lo interface,
it leads to incorrect calculation of UDP header
during UDP header compression (net_udp_get_hdr).

Fixes #10204

Signed-off-by: Johann Fischer <j.fischer@phytec.de>