net: udp: Remove NET_UDP_HDR() macro and direct access to net_buf

Remove NET_UDP_HDR() macro as we cannot safely access UDP header
via it if the network packet header spans over multiple net_buf
fragments.

Fixed also the UDP unit tests so that they pass correctly.

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