net/udp: Accept UDP packet with missing checksum

Having a checksum of zeros in UDP means "missing checksum" and is a
valid case as per RFC 768:

"An all zero transmitted checksum value means that the transmitter
generated no checksum (for debugging or for higher level protocols
that don't care)."

Such support is made possible by adding a new Kconfig option named
CONFIG_NET_UDP_MISSING_CHECKSUM.

However, that is valid only for IPv4. For IPv6, see the RFC 2460
section 8.1:

"Unlike IPv4, when UDP packets are originated by an IPv6 node, the UDP
checksum is not optional."

So the UDP checksum will always be verified in IPv6.

Fixes #16375

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>