Instead of defining a local version of struct udphdr use the standard
definition from <linux/udp.h>.

The 'src' field is named 'source' in the <linux/udp.h> definition.
Signed-off-by: NLuis Henriques <luis.henriques@canonical.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>