net: ipv6: Add implementation of main IPv6 functions

Functions that were exposed in "net: ipv6: Add IPv6 basic primitives"
had only empty implementations and were exposed as API for futher
patches. This patch add implementation of these functions. Main
functions are: net_ip6_handler() - IPv6 packet handler for incoming
packets; net_send_udp_packet6() - make up and send an UDP packet;
csum_ipv6_magic() - compute checksum of IPv6 "psuedo-header" per RFC2460
section 8.1; ip6_addr_in_subnet() - check if an address is in our
subnet. Other functions are auxiliary.

Series-changes: 3
- Added comments
- Fixed style problems
- Fixed return codes instead of -1

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>