net: Set used transport protocol to net_pkt

Cache the used transport protocol in net_pkt. This way we can
avoid traversing IP header to get the last protocol in network
packet. This is mostly an issue in IPv6 which can have a long
list of extension headers after IPv6 header and before the
transport protocol header.

Fixes #10853

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>